We present optical vortex beams interaction via degenerate two-wave mixing in liquid crystal light-valve, LCLV.
The LCLV is made by combining a transparent photosensitive substrate with a nematic liquid crystal and
displays a large Kerr-like effect. Being characterized by a large transverse size and spatial homogeneity, the
LCLV allows performing nonlinear mixing experiments with several vortex beams and testing the interaction
of different topological charges. We show that the wave-mixing occurring in the LCLV leads to the exchange
of topological charge between vortex and to a cascaded generation of vortex beams. A mean-field model is
developed and is shown to account for the charge selection rules observed after the mixing process. Fractional
charges are demonstrated to participate to the wave-mixing in a robust way, following the same charge selection
rules as for integer charges.
We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates irradiated by a
femtosecond Ti: Sapphire laser. In order to investigate the distribution of the refractive index change produced by the
femtosecond laser irradiation, we performed a two-dimensional visualization and spatially resolved optical analysis of
the induced refractive index profile by using a digital holographic technique and an adaptive-iterative algorithm for
wavefront reconstruction. The technique gives a direct and quantitative two-dimensional profile of the index of
refraction in irradiated samples, providing information how the fabrication process depends on the laser irradiation.
Digital holography in the mid infrared range is shown to be a feasible technique for optical metrological applications.
The technique allows to reconstruct both amplitude and phase of wavefronts scattered by a 3D object. Experimental
results of the method applied to the reconstruction of digitally holograms recorded at CO2 laser wavelength of 10.6
micron are reported. It is show that good reconstructions can be obtained even with the lower spatial resolution of IR
recording detectors compared to visible CCD array. The results show that new prospective can be exploited by using
high power CO2 laser sources in optical metrological applications.
A nonlinear optical medium results by the collective orientation of liquid crystal molecules tightly coupled to a
transparent photoconductive layer made of a BSO photorefractive crystal. The nonlinear medium, called photorefractive
liquid crystal light-valve, gives a large two-wave-mixing gain, thus, when inserted in a ring cavity, it results in an
unidirectional optical oscillator. Dynamical regimes with many interacting modes are made possible by the wide
transverse size and the high nonlinearity of the liquid crystal gain medium. In particular, we show the generation of
spatiotemporal pulses, coming from the random superposition of many longitudinal and transverse modes simultaneously
oscillating in the cavity.
A scientific problem described within a given code is mapped by a corresponding computational problem,
We call complexity (algorithmic) the bit length of the shortest instruction which solves the problem.
Deterministic chaos in general affects a dynamical systems making the corresponding problem
experimentally and computationally heavy, since one must reset the initial conditions at a rate higher than
that of information loss (Kolmogorov entropy). One can control chaos by adding to the system new degrees
of freedom (information swapping: information lost by chaos is replaced by that arising from the new
degrees of freedom). This implies a change of code, or a new augmented model.
Within a single code, changing hypotheses is equivalent to fixing different sets of control parameters, each
with a different a-priori probability, to be then confirmed and transformed to an a-posteriori probability via
Bayes theorem. Sequential application of Bayes rule is nothing else than the Darwinian strategy in
evolutionary biology. The sequence is a steepest ascent algorithm, which stops once maximum probability
has been reached. At this point the hypothesis exploration stops.
By changing code (and hence the set of relevant variables) one can start again to formulate new classes of
We call semantic complexity the number of accessible scientific codes, or models, that describe a situation.
It is however a fuzzy concept, in so far as this number changes due to interaction of the operator with the
system under investigation.
These considerations are illustrated with reference to a cognitive task, starting from synchronization of
neuron arrays in a perceptual area and tracing the putative path toward a model building.
In the last few years many non-destructive techniques have entered the field of painting conservation, and most of them are routinely applied to study and monitoring the painting status. Among them optical techniques are by now widely diffused and extremely well received because of their effectiveness and safety, nevertheless none of them is suitable for a quantitative characterization of varnish. One of the most important and often controversial stages of painting restoration is the surface cleaning process up to now being carried out without any tool to measure the actual varnish thickness but microscope observation of micro-detach. In this work we present an application of Optical Coherence Tomography to non-destructive diagnostics of artwork: the potentiality of this technique is demonstrated by measuring the thickness of the varnish layer in a fragment of a nineteenth-century oil painting.
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 (DSS), displays analogies with neurodynamic events which occur in the build-up of long term memories.
Homoclinic spike trains have been intensively investigated for single mode CO2 lasers; however, their occurrence has a more general significance insofar as this scenario fits the main aspects of action potentials in neurons. Stabilizing homoclinic trains has therefore a relevance for neural communication and synchronization, which seems to be the universal time code for perceptions. The core dynamics of homoclinic chaos is represented by the passage through a saddle point, in which neighborhood the system susceptibility (response to an external perturbation) is very high and hence it is very easy to apply a control. A few aspects of regularization of homoclinic chaos are covered, such as, synchronization by an external pace-maker, DSS (delayed self-synchronization), bursting, and NIS (noise induced synchronization). Such a general scenario is compared with specific neurodynamic models; moreover its impact on communicating with chaos is discussed. (Summary only available)
In this work we describe a simple method of encoding in real time information in the inter-spike intervals of a homoclinic chaotic system. This has been experimentally tested by means of an instantaneous synchronization between the laser intensity of a CO2 laser with feedback in the regime of homoclinic chaos and an external pulsed signal of very low power. The information is previously encoded in the temporal intervals between consecutive pulses of the external signal. The value of the inter-pulse intervals is varied each time a new pulse is generated. (Summary only available)
Many papers have been published recently about the characterization of time-dependent processes through techniques using wavelet approach. Our work takes into account a particular class of time-dependent processes in nonlinear realm. We want to characterize chaotic dynamics from the standpoint of its unstable periodicities. For this aim we introduce a new technique able to stabilize such unstable orbits. We illustrate this technique both from the theoretical and the experimental standpoint. As a further step, we want to deal with the problem of detecting and removing noise from chaotic dynamics. In this paper, firstly, we show how our technique is able to distinguish with very high sensitivity between a purely chaotic dynamics and a chaotic dynamics with noise even though the noise percentage is very low (of the order of 1 percent only Secondly, we apply our technique to remove noise from this dynamics. Finally, we compare both from the theoretical and experimental standpoint our technique with the well known wavelet technique. This work is a part of 'Skynnet' international project supported by the Italian National Institute for Nuclear Physics (INFN) and partially devoted to the application of new chaotic techniques instantiated in neural architectures for compressing, storing and transmitting information to earth from satellites.
Experimental evidence of periodic alternation of different transverse modes in a high-power CO2 laser is interpreted in terms of local cooling of the discharge column, which corresponds to the maximum emitted intensity. Optogalvanic coupling between emitted intensity and local discharge impedance values yields a transverse redistribution of discharge current and, hence, of laser gain. Thus, besides the fast feedback provided by the cavity, with high-power lasers one must account also for a slow global feedback because of the optogalvanic effects.
By increasing the Fresnel number of F of a ring cavity with photorefractive gain, the transition from a low F regime, where few modes compete in a regular or irregular sequence, to a high F regime, where many modes oscillate simultaneously giving rise to spatio-temporal chaos is shown. The transverse field is characterized by an increasing number of topological defects, whose mean separation is related to the spatial correlation length of the field.