Synchronization in semiconductor laser rings

We examine the dynamics of semiconductor lasers coupled in a ring configuration. The lasers, which have stable output intensity when isolated, behave chaotically when coupled unidirectionally in a closed chain. In this way, we show that neither feedback nor bidirectional coupling is necessary to induce chaotic dynamics at the laser output. We study the synchronization phenomena arising in this particular coupling architecture, and discuss its possible application to chaos-based communications. Next, we extend the study to bidirectional coupling and propose an appropriate technique to optical chaos encryption/decryption in closed chains of mutually coupled semiconductor lasers.Comment: 15 pages, 7 figure

Emergence of spike correlations in periodically forced excitable systems

In sensory neurons the presence of noise can facilitate the detection of weak information-carrying signals, which are encoded and transmitted via correlated sequences of spikes. Here we investigate relative temporal order in spike sequences induced by a subthreshold periodic input, in the presence of white Gaussian noise. To simulate the spikes, we use the FitzHugh-Nagumo model, and to investigate the output sequence of inter-spike intervals (ISIs), we use the symbolic method of ordinal analysis. We find different types of relative temporal order, in the form of preferred ordinal patterns which depend on both, the strength of the noise and the period of the input signal. We also demonstrate a resonance-like behavior, as certain periods and noise levels enhance temporal ordering in the ISI sequence, maximizing the probability of the preferred patterns. Our findings could be relevant for understanding the mechanisms underlying temporal coding, by which single sensory neurons represent in spike sequences the information about weak periodic stimuli

Numerical and experimental study of the effects of noise on the permutation entropy

We analyze the effects of noise on the permutation entropy of dynamical systems. We take as numerical examples the logistic map and the R\"ossler system. Upon varying the noise strengthfaster, we find a transition from an almost-deterministic regime, where the permutation entropy grows slower than linearly with the pattern dimension, to a noise-dominated regime, where the permutation entropy grows faster than linearly with the pattern dimension. We perform the same analysis on experimental time-series by considering the stochastic spiking output of a semiconductor laser with optical feedback. Because of the experimental conditions, the dynamics is found to be always in the noise-dominated regime. Nevertheless, the analysis allows to detect regularities of the underlying dynamics. By comparing the results of these three different examples, we discuss the possibility of determining from a time series whether the underlying dynamics is dominated by noise or not

Coherence and synchronization in diode-laser arrays with delayed global coupling

The dynamics of a semiconductor-laser array whose individual elements are coupled in a global way through an external mirror is numerically analysed. A coherent in-phase solution is seen to be preferred by the system at intermediate values of the feedback coupling strength. At low values of this parameter, a strong amplification of the spontaneous emission noise is observed. A tendency towards chaos synchronization is also observed at large values of the feedback strength.Comment: 8 pages, LaTeX, 6 PS figures, to appear in International Journal of Bifurcation and Chao

Frequency dynamics of gain-switched injection-locked semiconductor lasers

The frequency dynamics of gain-switched singlemode semiconductor lasers subject to optical injection is investigated. The requirements for low time jitter and reduced frequency chirp operation are studied as a function of the frequency mismatch between the master and slave lasers. Suppression of the power overshoot, typical during gain-switched operation, can be achieved for selected frequency detunings

Vivianite (ferrous phosphate) alleviates iron chlorosis in grapevine

Synthetic vivianite [Fe3(PO4)2·8H2O] has been reported to alleviate iron (Fe) chlorosis in crops growing on calcareous soils. To test the effectiveness of vivianite in grapevine we carried out three-year (2003 to 2005) experiments in vineyards located in six different areas of Spain with Denomination of Origin (Rioja, Ribera del Duero, La Mancha, Montilla-Moriles, Condado de Huelva, and Jerez), which differed in grapevine rootstock/variety, climate, and soil properties. In all cases there was at least one treatment in which a suspension of vivianite was injected into the soil at the beginning of the experiment in spring, one control (“–Fe”, no Fe fertilizer added) treatment, and one or more treatments with Fe chelate (FeEDDHA) or an Fe(II) salt applied yearly. The concentration of chlorophyll per unit leaf area was estimated with a portable chlorophyll meter (readings in SPAD units). The SPAD value and the trunk perimeter increment of the vines fertilized with vivianite were significantly higher than those of the control (-Fe) vines through the three years in all fields except the Jerez one. Vivianite was not significantly more effective than Fe-sulfate (in Rioja field) or Fe chelate (in La Mancha field). Our results suggest in summary that vivianite is effective in improving the Fe nutrition of vine and has a significant long-lasting effect of at least three years. This is ascribed to vivianite being incongruently dissolved to produce a poorly crystalline Fe oxide phase (lepidocrocite), which is considered to be a good source of Fe to plant. Vivianite is effective, readily prepared in the field, not easily leached from the soil, cheap, and environmentally safe, constituting thus an adequate Fe fertilizer for grapevine.

Zero-lag long-range synchronization via dynamical relaying

We show that simultaneous synchronization between two delay-coupled oscillators can be achieved by relaying the dynamics via a third mediating element, which surprisingly lags behind the synchronized outer elements. The zero-lag synchronization thus obtained is robust over a considerable parameter range. We substantiate our claims with experimental and numerical evidence of these synchronization solutions in a chain of three coupled semiconductor lasers with long inter-element coupling delays. The generality of the mechanism is validated in a neuronal model with the same coupling architecture. Thus, our results show that synchronized dynamical states can occur over long distances through relaying, without restriction by the amount of delay.Comment: 10 pages, 4 figure

Dynamics of modal power distribution in a multimode semiconductor laser with optical feedback

The dynamics of power distribution between longitudinal modes of a multimode semiconductor laser subjected to external optical feedback is experimentally analyzed in the low-frequency fluctuation regime. Power dropouts in the total light intensity are invariably accompanied by sudden activations of several longitudinal modes. These activations are seen not to be simultaneous to the dropouts, but to occur after them. The phenomenon is statistically analysed in a systematic way, and the corresponding delay is estimated.Comment: 3 pages, 4 figures, revte