In this paper we report the results of the research on a double side-pumped Er:Yb:glass slab laser. Due to the nonsymmetric power distribution inside the slab a displacement of the laser beam is observed thus leading to cavity
misalignment and consequently to extraction efficiency loss. At repetition rates of about 5Hz this allows just a burst
operation with an increased thermal load inside the slab. Numerical-theoretical studies about time-dependent temperature distribution in the laser material with appropriate boundary conditions concerning the considered asymmetric geometry lead to a model for mode axis thermal shift and allow to evaluate the amount of cavity misalignment in order to restore the maximal extraction efficiency. By turning or shifting the cavity mirrors is possible to improve the burst operation but the maximal overlap between the mode axis and the inverted region cannot be recovered. Then, an alternative optical configuration able to intrinsically account for the cavity misalignment has to be designed. The matrix formalism was effectively employed to calculate the specifics of one or more optical compensators to be inserted in the cavity to realign the laser resonator. Several configurations were analyzed and the beneficial effects on both thermal lensing and bending were predicted. Experimental measurements validated the model. In particular, an uninterrupted pulsed operation at the highest repetition rates was effectively recovered thus reducing the thermal load inside the laser slab and making such a laser system more effective in free-space laser applications.
The analytic solution to the problem of time-dependent non-homogeneous heat equation is derived for a
temporally pulsed and spatially non-homogeneous source term. For pulsed pumping with an inverse repetition rate much less than system thermal relaxation time, the problem of heat flow in a laser medium is typically studied within the approximation of time-independent heat equation. When the condition fails, due for instance to a short relaxation time or to a low repetition rate, transient analysis of thermal effects becomes necessary. Moreover the time-independent formalism fails in predicting both the focusing properties of the active material and any beam bending inside the resonator, while the transient analysis of thermal effects allows to finely predict the temperature distribution and to still apply, locally, the matrix formalism. In the paper we apply the formalism to a double side pumped Er:Yb glass slab laser with a non symmetric cooling scheme at several repetition rates. By evaluating the temporal evolution of the local temperature in the slab cross section, the difference with the stationary spatial temperature distribution turns out to be not negligible at repetition rates below 10 Hz. We observe that the lack of symmetry in the temperature profile reduces thermal focusing effects, but leads to a dynamic drift of mode laser axis which can make unstable the resonator cavity. We validated the model by comparing the theoretical values of slab focal length and of modal axis drift with experimental measurements at several repetition rates, proving also that the thermal focusing becomes a secondary effect in comparison with modal axis drift at increasing repetition rates.
An innovative Active Laser Imaging (ALI) vision system is presented. We report the experimental data of a short range
ALI able to achieve range measurements and to recognize people up to 5 km away, according to Johnson's criterion. We
also report the simulation data of a long range ALI working in excess of 10 km. The ALI uses a laser to illuminate an
area and a telescope to collect the scattered light into an InGaAs camera. The laser has been developed internally; it is an
eye safe class I pulsed laser operating at about 1.5 μm; its divergence and direction are changed according to the scene
and environmental conditions. The ALI can be used in active mode, with the laser on, or in passive mode using external
short wave infrared (SWIR) illumination sources. The data collected by the ALI and a thermal IR camera show the
ability of ALI to look across glasses and to read writings and the impossibility of thermal IR camera to do the same. We
describe the software models developed to emulate the ALI, the scenes, and the environmental conditions. The models
have been validated by experimental data and used to design the ALI.
The key features and performances of a compact, lightweight, high power Er<sup>3+</sup>:Yb<sup>3+</sup> glass laser transmitter are reported on. The theory employed to get an optimal design of the device is also described. In free running regime high energies of about 15mJ in 3ms long pulses were obtained, with an optical efficiency close to
85%. When q-switched by a Co: MALO crystal of carefully selected initial transmittivity, a high peak power in excess of
500 kW was obtained in about 9ns pulse duration, with an optical efficiency of 60%.
The laser was successfully run with no significant power losses at repetition rates up to 5Hz due to a carefully designed
heat sink which allowed an efficient conduction cooling of both the diode bars and the phosphate glass.
The transmitter emits at a wavelength of 1535nm in the
so-called "eyesafe" region of the light spectrum thus being
highly attractive for any application involving the risk of human injury as is typically the case in remote sensing
activities. Moreover, the spectral band around 1,5mm corresponds to a peak in the athmospheric transmittance thus being
more effective in adverse weather conditions with respect to other wavelengths.
Actually, the device has been successfully integrated into a rangefinder system allowing a reliable and precise detection
of small targets at distances up to 20Km. Moreover, the transmitter capabilities were used into a state of the art infrared
laser illuminator for night vision allowing even the recognition of a human being at distances in excess of 5Km.
The characterization of a plasma plume is a key issue in laser ablation and deposition studies. Combined diagnostic measurements by Optical Emission Spectroscopy (OES), ion time-of-flight (TOF) and Atomic Force Microscopy (AFM) have been used to study the dynamics and composition of laser ablation plume produced during ultrashort laser irradiation of metals and semiconductors, in vacuum. Our results show that, in the laser intensity range of 10<sup>12</sup>-10<sup>13</sup> W/cm<sup>2</sup>, the process of matter removal results in a plasma plume which is mainly composed of two different populations: atoms and nanoparticles. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atomic species have been identified by their characterstic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanparticles size distribution. In the case of silicon, the presence of a fast ion component emitted non-thermally from the sample surface as a result of the supercritical state induced by the intense ultrashort laser pulse irradiation has been also observed.