Biomedical subjects

R Meucci

Publications and source records attributed to R Meucci.

At least 19 recordsLinked to original sources

PubMed2005

Attractor selection in chaotic dynamics.

For different settings of a control parameter, a chaotic system can go from a region with two separate stable attractors (generalized bistability) to a crisis where a chaotic attractor expands, colliding with an unstable orbit. In the bistable regime jumps between independent attractors are mediated by external perturbations; above the crisis, the dynamics includes visits to regions formerly belonging to the unstable orbits and this appears as random bursts of amplitude jumps. We introduce a control method which suppresses the jumps in both cases by filtering the specific frequency content of one of the two dynamical objects. The method is tested both in a model and in a real experiment with a CO2 laser.

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PubMed2005

Dynamics and stabilization of the Elettra storage-ring free-electron laser.

The ultimate performance of a storage-ring free-electron laser in terms of light stability and extracted power depends on the possibility of simultaneously controlling the electron-beam and laser dynamics. As a preliminary requirement, the level of longitudinal and transverse electron-beam stability must be high enough to guarantee the laser start-up and growth. This is usually obtained by means of dedicated feedback systems. Once such a requirement is satisfied, the possibility of establishing and maintaining a continuous-wave operation mode finally resides in a deep understanding of the strongly coupled laser-electrons dynamics. For this purpose, we have developed a simple theoretical model which has been proved to be able to provide insight into the evolution of the laser intensity. In this framework, we have also proposed the possibility of utilizing a derivative closed-loop feedback to create or enlarge the region of stable signal. A feedback of this type has been implemented on the Elettra storage-ring free-electron laser. The obtained results, which fully confirm our predictions, are discussed in this paper.

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PubMed2005

Coherence resonance in excitable electronic circuits in the presence of colored noise.

We give evidence of coherence resonance in an excitable electronic circuit whose dynamics obeys the FitzHugh-Nagumo model system, under the application of different noise sources, ranging from Gaussian white noise to colored 1/f2 noises. The resonance behavior can be significantly reinforced when experimental parameters are tuned in order to place the stable fixed point closer to the excitability threshold of spiking behavior, as well as when the time scales of the circuit are properly modified. A quantitative description of the effects of noise correlations in inducing the resonant behavior is provided.

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PubMed2005

A programmable electronic circuit for modelling CO2 laser dynamics.

We introduce a programmable electronic circuit implementing the rich dynamics of CO2 laser models. The design and the implementation of the circuit are accomplished by using a programmable analog device, which permits an experimental characterization of the laser dynamics. The experimental results shown in the paper demonstrate that the circuit exhibits homoclinic chaos typical of CO2 laser with feedback modulation of cavity losses. Moreover, experimental results showing that noise regularizes the dynamical time scales of the system are reported.

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PubMed2004

Coupling scheme for complete synchronization of periodically forced chaotic CO2 lasers.

We present a way of coupling two nonautonomous, periodically forced, chaotic C O2 lasers in a master-slave configuration in order to achieve complete synchronization. The method consists of modulating the forcing of the slave laser by means of the difference between the intensities of the two lasers, and lends itself to a simple physical implementation. Experimental evidence of complete synchronization induced by a suitable coupling strength is shown, and a numerical model is used to achieve further insight of the synchronization phenomena. Finally, we describe a possible application of the investigated technique to the design of a digital communication system.

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PubMed2004

Experimental control of coherence of a chaotic oscillator.

We give experimental evidence that a delayed feedback control strategy is able to efficiently enhance the coherence of an experimental self-sustained chaotic oscillator obtained from a CO2 laser with electro-optical feedback. We demonstrate that coherence control is achieved for various choices of the delay time in the feedback control, including values that would lead to the stabilization of an unstable periodic orbit embedded within the chaotic attractor. The relationship between the two processes is discussed.

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PubMed2003

Constructive effects of noise in homoclinic chaotic systems.

Many chaotic oscillators have coherent phase dynamics but strong fluctuations in the amplitudes. At variance with such a behavior, homoclinic chaos is characterized by quite regular spikes but strong fluctuation in their time intervals due to the chaotic recurrence to a saddle point. We study influences of noise on homoclinic chaos. We demonstrate both numerically and experimentally on a CO2 laser various constructive effects of noise, including coherence resonance, noise-induced synchronization in uncoupled systems and noise-enhanced phase synchronization, deterministic resonance with respect to signal frequency, and stochastic resonance versus noise intensity in response to weak signals. The peculiar sensitivity of the system along the weak unstable manifold of the saddle point underlines the unified mechanism of these nontrivial and constructive noise-induced phenomena.

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PubMed2003

Noise-enhanced synchronization of homoclinic chaos in a CO2 laser.

Many chaotic oscillators have rather coherent phase dynamics but strong fluctuation in the amplitudes. Conversely, homoclinic chaos is characterized by quite regular spikes but strong fluctuation in their time intervals. We study the effects of noise on the synchronization of homoclinic chaos to a weak periodic signal and demonstrate numerically and experimentally in a CO2 laser system that noise enhances synchronization of homoclinic chaos. The system exhibits both conventional resonance versus driving frequency and stochastic resonance with respect to noise intensity.

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PubMed2003

Information encoding in homoclinic chaotic systems.

We present a simple method for real-time encoding of information in the interspike intervals of a homoclinic chaotic system. The method has been experimentally tested on a CO2 laser with feedback displaying Sil'nikov chaos and synchronized with an external pulsed signal. Information is encoded by the length of the temporal intervals between consecutive pulses of the external signal. This length is varied each time a new pulse is generated.

Carbon Dioxide↗

PubMed2003

Controlling transient dynamics to communicate with homoclinic chaos.

A control that stabilizes the transient dynamics of a homoclinic chaotic laser is used to encode discrete sources of information. The controlled trajectory is a complex spiking signal that has a constrained interspike interval, and therefore, the ratio of information transmitted is approximately constant. We also show that the controlled signal that encodes the source contains more information than the source. This property is advantageously used to correct possible errors in the transmission, or to increase the ratio of information per transmitted spike.

Carbon Dioxide↗

PubMed2003

Factors influencing pricking pain threshold using a CO2 laser.

The purpose of this study was the evaluation of the best experimental conditions in healthy subjects for the measurement of the minimal thermal energy density E1 which induced pricking pain on the volar surface of the left forearm by means of CO2 laser pulses. E1 was measured on a well-defined area, using laser pulses of different durations and constant power P. The dependence of E1 on the stimulus power P, the size A of the radiated area and the surface temperature T(e) were explored. In the first part of the study, these relations were obtained using a computer program, from the calculated spatio-temporal distribution of the skin temperature during, and following, a laser pulse which caused pricking pain. The second part studied a set of subsequent measurements carried out on a group of five healthy trained subjects and agreed only in part with the calculated data. We found that the measurement error on E(t) was less than 10% with P between 1.5 and 3 W, and A between 0.15 and 0.25 cm2, respectively. The influence of sensitization and adaptation phenomena on the measured data was also explored. We also show a rhythmic annual change of T(e) and E1.

Adult↗

PubMed2002

Experimental characterization of the transition to phase synchronization of chaotic CO2 laser systems.

We investigate the transition route to phase synchronization in a chaotic laser with external modulation. Such a transition is characterized by the presence of a regime of periodic phase synchronization, in which phase slips occur with maximal coherence in the phase difference between output signal and external modulation. We provide the first experimental evidence of such a regime and demonstrate that it occurs at the crossover point between two different scaling laws of the intermittent-type behavior of phase slips.

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PubMed2002

Chaotic function generator: complex dynamics and its control in a loss-modulated Nd:YAG laser.

The complex dynamics resulting from electronic feedback of a laser's intensity are explored and characterized. Distinct stable and chaotic regimes can be elicited from the laser by tuning the bias of the feedback loop. An additional branch of the feedback loop, containing a derivative filter, provides access to new kinds of dynamics, including a more gradual transition to chaos. The whole feedback network together allows the laser dynamics to be selected from among a wide range of chaotic wave forms distinguished by statistical or spectral information. In other words, this laser system can be used as a tunable generator of chaotic functions.

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PubMed2002

Delayed self-synchronization in homoclinic chaos.

The chaotic spike train of a homoclinic dynamical system is self-synchronized by applying a time-delayed correction proportional to the laser output intensity. Due to the sensitive nature of the homoclinic chaos to external perturbations, stabilization of very long-periodic orbits is possible. On these orbits, the dynamics appears chaotic over a finite time, but then it repeats with a recurrence time that is slightly longer than the delay time. The effect, called delayed self-synchronization, displays analogies with neurodynamic events that occur in the buildup of long-term memories.

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PubMed2002

Autonomous bursting in a homoclinic system.

The output of a dynamical system in a regime of homoclinic chaos transforms from a continuous train of irregularly spaced spikes to clusters of regularly spaced spikes with quiescent periods in between (bursting), provided a low frequency portion of the output is fed back. We provide experimental evidence of such an autonomous bursting by a CO2 laser with feedback. The phenomena here presented are extremely robust against noise and display qualitative analogies with bursting phenomena in neurons.

Action Potentials↗

PubMed2001

Identifying physical properties of a CO2 laser by dynamical modeling of measured time series.

We estimate internal parameters of a Q-switched CO2 laser by fitting trajectories of the four-level model to measured scalar time series. The four-level model is a five-dimensional nonlinear system of ordinary differential equations. A multiple shooting technique is used to construct the unobserved time courses of the population densities and to reveal the dependence of the parameters on the excitation current. For excitations barely above the laser threshold large pulse variations are identified as an effect of small variations of the pump parameter.

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PubMed2001

Synchronization of homoclinic chaos.

Homoclinic chaos is characterized by regular geometric orbits occurring at erratic times. Phase synchronization at the average repetition frequency is achieved by a tiny periodic modulation of a control parameter. An experiment has been carried on a CO(2) laser with feedback, set in a parameter range where homoclinic chaos occurs. Any offset of the modulation frequency from the average induces phase slips over long times. Perfect phase synchronization is recovered by slow changes of the modulation frequency based upon the sign and amplitude of the slip rate. Satellite synchronization regimes are also realized, with variable numbers of homoclinic spikes per period of the modulation.

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PubMed2000

Discrete homoclinic orbits in a laser with feedback

We provide experimental evidence of the discrete character of homoclinic chaos in a laser with feedback. We show that the narrow chaotic windows are distributed exponentially as a function of a control parameter. The number of consecutive chaotic regions corresponds to the number of loops around the saddle focus responsible for Shilnikov chaos. The characterization of homoclinic chaos is also done through the return map of the return times at a suitable reference point.

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