Alessandro Scirè

Operating regimes of GaAs-AlGaAs semiconductor ring lasers: experiment and model

Theory and experiments of single-mode ridge waveguide GaAs-AlGaAs semiconductor ring lasers are presented. The lasers are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Bidirectional operation reveals that just above threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave. As the injected current is increased, a new regime appears where the intensities of the two counterpropagating modes undergo alternate sinusoidal oscillations with frequency in the tens of megahertz range. The regime with alternate oscillations was previously observed in ring lasers of the gas and dye type, and it is here reported and investigated in semiconductor ring lasers. A theoretical model based on a mean field approach for the two counterpropagating modes is proposed to study the semiconductor ring laser dynamics. Numerical results are in agreement with the regime sequence experimentally observed when the injected current is increased (i.e., bidirectional continuous-wave, bidirectional with alternate oscillations, unidirectional). The boundaries of the different regimes are studied as a function of the relevant parameters, which turn out to be the pump current and the conservative and dissipative scattering coefficients, responsible for an explicit linear coupling between the two counterpropagating field modes. By a fitting procedure, we obtain good numerical agreement between experiment and theory, and also an estimation for the otherwise unknown scattering parameters.

Dynamic behavior and locking of a semiconductor laser subjected to external injection

In this paper, we analyze the phenomena arising when a monomode semiconductor laser is subjected to external injection from another laser. The system stability is investigated as a function of detuning and of the relative injected power. Different regimes, spanning from phase locking to chaos and coherence collapse, are described by analytical and numerical methods for weak and moderate injection. Previous theoretical studies are extended by describing the inverse transition from chaos to stability and by deriving the final locking condition. Also, further investigation on the coherence collapse regime has been performed. Besides contributing to the exploration of an interesting fundamental phenomenon, the results of this analysis are useful for different applications, including coherent detection and chaotic cryptography.

Synchronization of chaotic lasers by optical feedback for cryptographic applications

We propose a new scheme for synchronization of the optical chaos generated by a semiconductor laser subjected to external reflection. The scheme is based on optical feedback and will be analyzed from the viewpoint of static and dynamic properties and of robustness to external perturbations and noise. An application to cryptographic communications (chaotic shift keying) is finally proposed.

Alternate oscillations in semiconductor ring lasers

We report on fabrication and characterization of single-longitudinal- and transverse-mode semiconductor ring lasers. A bifurcation from bidirectional stable operation to a regime with alternate oscillations of the counterpropagating modes was observed experimentally and is theoretically explained through a two-mode model. Analytical expressions for the onset and the frequency of the oscillations are derived, and L-I curves numerically evaluated. Good quantitative agreement between theory and measurements made over a large number of tested devices is obtained.

Bistability and all-optical switching in semiconductor ring lasers

Semiconductor ring lasers display a variety of dynamical regimes originating from the nonlinear competition between the clockwise and counter-clockwise propagating modes. In particular, for large pumping the system has a bistable regime in which two stationary quasi-unidirectional counter-propagating modes coexist. Bistability is induced by cross-gain saturation of the two counter-propagating modes being stronger than the self-saturation and can be used for data storage when the semiconductor ring laser is addressed with an optical pulse. In this work we study the response time when an optical pulse is injected in order to make the system switch from one mode to the counter-propagating one. We also determine the optimal pulse energy to induce switching.

Two-dimensional phase-space analysis and bifurcation study of the dynamical behaviour of a semiconductor ring laser

A basic rate equation model of a quantum-well semiconductor ring laser is reduced to two equations using asymptotic methods. The reduced model allows for analytical expressions of the bifurcation points, which will simplify future model parameter estimations, and motivates a two-dimensional phase-space description of the dynamical behaviour. An analysis of the bifurcation scenarios in different parameter regimes is pursued. Physical conditions for the emergence of the operating regimes are assessed quantitatively in terms of saturation processes and backscattering mechanisms.

Theory of collective firing induced by noise or diversity in excitable media

Large variety of physical, chemical, and biological systems show excitable behavior, characterized by a nonlinear response under external perturbations: only perturbations exceeding a threshold induce a full system response (firing). It has been reported that in coupled excitable identical systems noise may induce the simultaneous firing of a macroscopic fraction of units. However, a comprehensive understanding of the role of noise and that of natural diversity present in realistic systems is still lacking. Here we develop a theory for the emergence of collective firings in nonidentical excitable systems subject to noise. Three different dynamical regimes arise: subthreshold motion, where all elements remain confined near the fixed point; coherent pulsations, where a macroscopic fraction fire simultaneously; and incoherent pulsations, where units fire in a disordered fashion. We also show that the mechanism for collective firing is generic: it arises from degradation of entrainment originated either by noise or by diversity.

Topological Insight into the Non-Arrhenius Mode Hopping of Semiconductor Ring Lasers

We investigate both theoretically and experimentally the stochastic switching between two counterpropagating lasing modes of a semiconductor ring laser. Experimentally, the residence time distribution cannot be described by a simple one-parameter Arrhenius exponential law and reveals the presence of two different mode-hop scenarios with distinct time scales. In order to elucidate the origin of these two time scales, we propose a topological approach based on a two-dimensional dynamical system.

Modal structure, directional and wavelength jumps of integrated semiconductor ring lasers: Experiment and theory

We have experimentally and theoretically analyzed the modal properties of semiconductor ring lasers and the wavelength jumps that occur in connection with directional switching above the threshold.

Characterization of a chaotic telecommunication laser for different fiber cavity lengths

In this paper, we consider a single-mode telecommunication laser emitting at 1550 nm, which is subjected to backreflection from a mirror at the end of a long fiber cavity. In view of cryptographic applications, we focus our analysis on the chaotic regime, which we investigate as a function of pump current, backreflection level, and cavity length. Experimental data are compared with numerical simulations, showing the advantages of the fiber approach.