The feasibility of the cascaded second and third harmonic generation in two-sectioned periodically
poled lithium tantalate crystal is analyzed. Simulation using computational non-linear optical model
rigorously coupled with the thermal model suggests that 20-30 % efficiency can be achieved for 3 W
power 2.2 ns pulsed 1.064 μm laser operating at frequency 6.8x10<sup>3</sup> Hz if the crystal is composed with
optimized section lengths for: (i) 8.0 μm periodic first-order SHG structure and (ii) 6.6 μm periodic
third-order THG structure. Significant inhibition of THG efficiency can be due to the heat release of SH
and TH along the crystal, associated thermal dephasing and lenzing which can be effectively inhibited by
decreasing the crystal cross-section dimensions to the practical minimum of 200x200 μm.
Polycrystalline ceramic with activator ions has already become popular material as laser medium.
In this study, composite type rod which consisted with doped- and undoped-YAG sections was
prepared for the laser experiments. By the improvement of pumping chamber, pulse energy of 930
mJ was obtained from oscillator with TEM00 in normal pulse mode at 10 Hz at room temperature.
Giant pulse generation was carried out by inserting an AO Q-switch into the laser cavity. Because of
the limitation of surface damage thresholds on the laser rod and the mirrors, the maximum output
energy up to 21 mJ was obtained.
Integrated computational model for operation of co-doped Tm,Ho solid-state lasers is developed coupling (i) 8-level rate equations with (ii) TEM00 laser beam distribution, and (iii) complex heat dissipation model. Simulations done for Q-switched ≈0.1 J giant pulse generation by Tm,Ho:YLF laser show that ≈43 % of the 780 nm light diode side-pumped energy is directly transformed into the heat inside the crystal, whereas ≈45 % is the spontaneously emitted radiation from <sup>3</sup>F<sub>4</sub>, <sup>5</sup>I<sub>7</sub> , <sup>3</sup>H<sub>4</sub> and <sup>3</sup>H<sub>5</sub> levels. In water-cooled operation this radiation is absorbed inside the thermal boundary layer where the heat transfer is dominated by heat conduction. In high-power operation the resulting temperature increase is shown to lead to (i) significant decrease in giant pulse energy and (ii) thermal lensing.
Formation of nano-fibers ranging from 30 to 300 nm in diameter and exceeding the length of one millimeter was
observed during explosive ablation of As<sub>2</sub>S<sub>3</sub> glass by femtosecond pulses at high fluence (> 5 J/cm<sup>2</sup>) irradiation of the 800 nm wavelength, 160 fs duration pulses in air. However, the spheres of up to several microns in diameter are found to
form competing with nano-fiber formation and significantly deteriorating their morphology. The formation of these
spheres is explained by the free energy minimization of explosively ablating liquid jets combined with the onset and
evolution of the thermo-capillary forces at the local perturbations of the geometry and temperature. Performed thermal
analysis suggests that the good nano-fiber morphology can be preserved by increasing the air pressure or ablation in
water which shortens the characteristic cooling and solidification time of the liquid jets, and inhibits the development of
the small geometrical perturbations into the large spheres.
Computational study of nanosecond pulse laser radiation in periodically poled LiNbO<sub>3</sub> and LiTaO<sub>3</sub> crystals reveals the complex spacio-temporal evolution of the 1.064 μm fundamental harmonic (FH) and second harmonic (SH) energy fields with associated temperature fields, leading to the thermal dephasing and inhibition of second harmonic generation (SHG). The investigated range of the laser input power is W<sub>0</sub>=0.5-50 W (with the pulse energy Q<sub>0</sub>=0.01-1 mJ/pulse and repetition rate of 50 kHz). For input laser powers W<sub>0</sub>>10 W the FH and SH energy fields are found to strongly couple with non-uniform temperature field leading to significant thermal dephasing and SHG efficiency loss. Heat generation and temperature distributions also exhibit very significant non-uniformities along and across the laser beam, maximizing at the rear or inside the crystal, depending on the input power. Performed study shows the feasibility of the effective thermal control with temperature gradient along the crystal allowing one to maintain (i) the irradiated zone within the temperature tolerance range and (ii) high SHG efficiency under high input laser powers.