Marco Ciofini

THE CONTROL OF CHAOS: THEORETICAL SCHEMES AND EXPERIMENTAL REALIZATIONS

Controlling chaos is a process wherein an unstable periodic orbit embedded in a chaotic attractor is stabilized by means of tiny perturbations of the system. These perturbations imply goal oriented feedback techniques which act either on the state variables of the system or on the control parameters. We review some theoretical schemes and experimental implementations for the control of chaos.

3-D numerical simulation of Yb:YAG active slabs with longitudinal doping gradient for thermal load effects assessment.

We present a study of Yb:YAG active media slabs, based on a ceramic layered structure with different doping levels. We developed a procedure allowing 3D numerical analysis of the slab optical properties as a consequence of the thermal load induced by the pump process. The simulations are compared with a set of experimental results in order to validate the procedure. These structured ceramics appear promising in appropriate geometrical configurations, and thus are intended to be applied in the construction of High Energy Diode Pumped Solid State Laser (DPSSL) systems working in high repetition-rate pulsed regimes.

Efficiency optimization for a diode-pumped Nd:YAG ceramic slab laser

With the help of photometric calculations based on ray-tracing algorithms, we have optimized the efficiency of the optical pumping of a Nd:YAG ceramic slab laser. The slab pumping is performed by means of two horizontal diode laser array stacks. The use of two small reflecting walls allows the sort of duct coupling that is capable of significantly improving the performance of the system. Our first experiments with a simple direct coupling provided a maximum extraction of slightly more than 160 W at a 20% slope efficiency level. The use of the optimized short duct coupling leads us to the extraction of 350 W with a slope efficiency of 51%, making use of the same diode arrays. The laser design is suitable for the construction of cw sources with a power output above 1 kW.

Compact scalable diode-pumped Nd:YAG ceramic slab laser

We describe our preliminary studies of the use of neodymium-doped slab-shaped ceramic YAG media in the construction of compact, rugged, high-power diode-pumped solid-state lasers. A maximum extraction of more than 160 W at a 20% slope efficiency level, with a narrow transverse direction beam-parameter product of the order of 4 mm mrad(-1) is experimentally obtained from an extremely simple and compact (overall dimensions 160 mm x 100 mm x 60 mm) laser head in a quasi-continuous-wave regime. Experimental data together with finite-elements method simulations indicate that power extraction can be scaled up at least to 900 W cw with this laser head geometry.

Guided Talbot resonators for annular laser sources

We studied the properties of an annular waveguide laser resonator, performing a new set of numerical simulations. The existence of fractional-Talbot effects is shown and results are compared to previous free-space propagation calculations and experimental results. Our study allows us to design annular waveguide lasers efficiently extracting power into a well behaving field distribution, convertible into a high quality beam by means of simple optics.

Extraction of high-quality beams from narrow annular laser sources

We have analyzed the propagation properties of the field produced by circular arrays of coherent optical sources, obtaining the conditions for the generation of good-quality global beams. Such conditions can be obtained by the use of resonators based on the Talbot effect. This appears to be a practical method for the construction of simple, low-cost resonators for compact diffusion-cooled high-power lasers with annular format. Low-loss annular Talbot cavity configurations have also been studied.

Polarization state modifications in the propagation of high azimuthal order annular beams

Using a vector Fresnel diffraction propagator we investigate the far-field distributions obtained from guided annular modes with different polarization states. Furthermore we demonstrate that a pure azimuthal polarization transforms into a mainly radial one in the propagation of annular beams with azimuthal mode number higher than 0. This property could enhance the performance of a laser metal-cutting system based on these kind of beams.

Beam quality enhancement for a radio-frequency excited annular CO2 laser

We report the preliminary experimental results obtained from an annular radio-frequency excited CO2 laser with a Talbot cavity and a phase correcting mirror external to the cavity. The Talbot cavity is adopted in order to reduce the number of oscillating azimuthal modes. The use of the external profile-modulated mirror is aimed at reducing the beam phase modulation and thus increasing the amount of fundamental annular mode energy content. The beam quality results to be sensibly increased.

Polarization states of annular pattern in a nearly-isotropic laser resonator

Abstract We investigate the polarization properties of the annular pattern in a CO 2 laser designed to preserve a good cylindrical symmetry. We found that this pattern can show two different polarization states. The first one corresponds to a superposition of two independent Gauss-Hermite modes with linear and orthogonal polarizations. The second one is a linearly polarized state. A simple model, including anisotropies both for the cavity losses and the detuning, can explain the observed transition between these two states. We also provide an interpretation of the interplay between polarization states and corresponding spatial patterns.

Laser and optical properties of Yb:YAG ceramics with layered doping distribution: design, characterization and evaluation of different production processes

The laser, optical and spectroscopic properties of multilayer Yb:YAG ceramic structures, differently activated, were investigated. The structures were designed by means of Finite Element Modeling, adjusting the doping distributions to reduce peak temperature, surface deformation and thermally induced stresses, depending on the pump and cooling geometry. Two ceramic processes were used, i.e. dry pressing of spray-dried powders (SD) and tape casting (TC), resulting in different defect density and size distribution: TC gives a more uniform transmission, whereas SD results in larger, unevenly scattered defects. The spectroscopic properties were found independent from the production process. The laser performance has been characterized under high intensity pumping in a longitudinally diode pumped laser cavity, comparing the behavior of the different structures in terms of slope efficiency, stability under increasing thermal load, spatial uniformity of laser emission. Slope efficiency values as high as 58% in Quasi-CW pumping conditions and 54% in CW conditions was measured in two-layers structures. The production process and the number of layers influenced the behavior of the samples, in particular regarding the spatial uniformity of the laser emission. Samples made by tape casting have shown overall a better thermal stability with respect to the samples made by spray drying.