Home – Home – Jornals – Advanced Search

The process of second harmonic generation (SHG) of laser radiation in a BBO crystal is theoretically studied with the aim of determining optimal values of focusing and wave detuning parameters providing maximal SHG efficiency. This optimization problem is solved under an additional condition, that is, the maximal radiation power density should not exceed an a priori specified value. No restrictions are posed on the radiation power. The study consists of two parts. This is the second part of the work, where the influence of the additional condition on the process and results of solution of the optimization problem is studied using rigorous methods for solution of a system of nonlinear wave equations. The main results of the studies are the dependences of the maximum attainable SHG efficiency on the laser radiation power. It is shown that the optimal values of focusing and wave detuning parameters can be found with the use of the preset field approximation, which significantly reduces the amount of calculations, in many practical cases.

The effect of the electrode material (aluminum and stainless steel) on the generation of mini-jets on the color of a pulse-periodic diffuse discharge in air, nitrogen, and argon is studied. A discharge was generated under generation of runaway electrons in an inhomogeneous electric field. It is found that the material of the electrodes significantly influences the color of the mini jets that originate when bright spots appear on the electrodes. It is confirmed that the use of aluminum electrodes colors the mini jets red, and of iron electrodes, blue. It is shown that the color of the discharge plasma in the mini jet region corresponds to the color of high-altitude atmospheric discharges (red sprites and blue jets) and differs from the color of diffuse discharges in air and nitrogen under the same pressure.

The emission characteristics of a small-size XeCl laser (discharge plasma volume is ~ 10 cm³) are calculated based on a model taking into account specific features of operation of the excitation system, active medium, and cavity. Good agreement with experimental results is obtained.

An electric-discharge XeCl mini laser and the results of its experimental modeling are described. The laser uses a two-stage system of magnetic compression of current pulses, semiconductor high-voltage bipolar transistors as a switch, and preionization of the working medium by barrier discharge radiation from under the grid electrode. The dependence of the output parameters of the mini laser on the conditions of its excitation and the composition of the active medium is established. The maximal pulse energy of ~ 12 mJ was obtained for a mixture of the composition 1.5 torr HCl + 15 torr Xe + 4 atm Ne at a voltage across the peaking capacitor of 20 kV. The duration of the lasing pulses at half maximum was ~ 20 ns. The efficiency of the XeCl mini laser relative to the energy stored in the sharpening capacitor is ~ 4%.

The results of recent studies aimed at increasing the energy and peak power of THL-100 hybrid femtosecond laser system in visible range based on photodissociation XeF(C-A) amplifier and Ti:Sa femtosecond complex are presented. Two versions of the laser system operation based on the amplification of positively or negatively chirped pulses in the XeF(C-A) amplifier are analyzed. It is shown that when the positive-chirped pulse is amplified, the radiation energy maximum is achieved, and when the negative-chirped pulse is amplified, the power maximum is achieved.

Results of laboratory experiments on control of the filamentation domain of a focused femtosecond laser beam with a spatially structured (using a deformable mirror) wavefront are described. Aberration beams consisting of coherent interleaved ring subapertures obtained with a deformable mirror of specific form are studied. This allows producing inhomogeneities of the distribution of the optical field amplitude during laser beam propagation; these inhomogeneities can subsequently become seeds for light filaments. This approach to control high-power radiation filamentation does not require compensation for distortions of the initial beam profile; on the contrary, it is based on controllable introduction of pre-calculated wavefront aberrations.

Economically motivated adulterations (EMAs) of food and juice are a serious threat to our health. Although several accurate analytical methods are available to detect fraudulent ingredients in the supply chain, fast and user-friendly techniques are still missing, especially if reliable deployment in industrial settings is needed. After many years of application of laser photoacoustic spectroscopy (LPAS) to food fraud detection with CO₂ lasers, the Diagnostic and Metrology Laboratory of ENEA developed a portable and robust prototype based on a quantum cascade laser (QCL) to rapidly and easily identify EMAs in real scenarios. Fruit juice was used as a case study to evaluate its performance. Two EMAs were sensed in a few minutes and chemometrics tools allowed their quantification

An important characteristic of a natural reservoir is the distribution of the pigment composition of various groups of photosynthetic organisms over depth and its changes in different seasons. Optical methods are widely used to monitor the concentration of chlorophylls of algae or cyanobacteria in aquatic ecosystems, but they are very limited used to monitor the bacteriochlorophyll (BChl) concentration of phototrophic bacteria. The paper describes in detail three methods of quantitative analysis of the photosynthetic pigment of BChl d of green sulfur bacteria using spectral measurements both in initial samples of natural water (optical density spectra) and in pigment extracts in organic solvents (absorption and fluorescence spectra). A comparative analysis of the results of different methods is carried out on the example of natural water from the chemocline region of the lake Trekhtzvetnoe, one of the meromictic reservoirs of the Kandalaksha Bay of the White Sea (water from different depths was selected in March 2021). The conclusions about the conditions of applicability of different methods, their advantages and limitations were made. All three methods for estimating the BChl concentration have proved their applicability on the samples of natural water with BChl concentrations differing by orders of magnitude, from 5 to 5000 mg/m³.

Results of studies of the horizontal orientation of crystalline particles carried out using a scanning polarization lidar LOSA-M3 are presented During 2018-2021, several series of measurements of the structure of high-level crystalline clouds were carried out in the zenith scanning mode. In contrast to sounding only in the vertical direction, observations of the dependence of the lidar signal characteristics (intensity and depolarization ratio) on the angle of lidar axis inclination make it possible to identify the phase composition of clouds (water or crystalline) and measure the distribution of particle deviation relative to the horizontal plane (flutter). In layers with a pronounced specular reflection, the relationship between the signal intensity and the slope of the sounding path at small angles (up to 5°) is well described by an exponential dependence. The results of sounding when scanning up to angles of 45-50° showed a high probability of the existence of a corner reflection in ice clouds.

Generation of a quasi-neutral beam of protons and electrons during the interaction of an ultra-high intensity laser pulse with an aluminum target with a layer of protons on the rear surface is studied. It is shown that the time of setting the quasi-neutrality of the proton beam is more than five time longer than the laser pulse duration.