An ultrashort-pulse mid-infrared laser system tunable within a wavelength range of similar to 5-11 mu m is experimentally demonstrated. The system is based on difference frequency generation in a nonlinear crystal, where a Ti:Sapphire pump beam is mixed with a signal pulse formed through self-phase modulation of the Ti:Sapphire laser pulse under its filamentation in a CO2 gas cell. Three nonlinear crystals were examined in this setup: ZnGeP2, AgGaS2, and LiGaS2. Central wavelength tuning from similar to 5.5 mu m to similar to 10 mu m was carried out with AgGaS2 and LiGaS2 crystals. The LiGaS2 crystal provided the highest mid- infrared pulse energy of 2.7 mu J at 6 mu m wavelength and broadband pulse spectrum within 7.5-11.5 mu m wavelengths. The ZnGeP2 crystal provided the highest efficiency of 0.16 % for frequency conversion into similar to 10 mu m wavelength region, but that took place at low pump pulse energy/intensity (0.3 mJ/similar to 30 GW/cm(2)), which, being higher, resulted in lower efficiency due to nonlinear absorption.

The efficiency of AgGaS2, BaGa4Se7, and HgGa2S4 crystals has been investigated in a mid-IR frequency difference generator based on a femtosecond ytterbium fiber laser with a wavelength of 1.03 μm, pulse duration of 0.25 ps, pulse repetition rate of 10 kHz, and average radiation power of 5 W. It is shown that under the given experimental conditions the highest efficiency of difference-frequency generation (up to 7.5

The sum-frequency generation of broadband CO laser radiation (λ ≈ 5–6 μm) in a new nonlinear BaGa2GeS6 crystal at type I and II phase matching at the variation of polar and azimuth angles of this crystal has been experimentally investigated. A small mismatch ( 3°) between the values of measured and calculated phase matching angles was found. Also, based on experimental data and their comparison with calculations, a set of coefficients was determined of the nonlinear susceptibility tensor of BaGa2GeS6, which best describes the experimental results.

An ultrashort-pulse mid-infrared laser system tunable within a wavelength range of ∼ 5–11 μm is experimentally demonstrated. The system is based on difference frequency generation in a nonlinear crystal, where a Ti:Sapphire pump beam is mixed with a signal pulse formed through self-phase modulation of the Ti:Sapphire laser pulse under its filamentation in a CO2 gas cell. Three nonlinear crystals were examined in this setup: ZnGeP2, AgGaS2, and LiGaS2. Central wavelength tuning from ∼ 5.5 μm to ∼ 10 μm was carried out with AgGaS2 and LiGaS2 crystals. The LiGaS2 crystal provided the highest mid- infrared pulse energy of 2.7 μJ at 6 μm wavelength and broadband pulse spectrum within 7.5–11.5 μm wavelengths. The ZnGeP2 crystal provided the highest efficiency of 0.16 % for frequency conversion into ∼ 10 μm wavelength region, but that took place at low pump pulse energy/intensity (0.3 mJ/∼30 GW/cm2), which, being higher, resulted in lower efficiency due to nonlinear absorption.

Generation of ultrashort mid-IR pulses spanning from 8.5 to 10.5 μm with high energy (up to 4.5 μJ) was experimentally demonstrated through difference frequency generation in BaGa 2 GeSe 6 crystal pumped by 100-fs 0.95-μm Ti:sapphire laser pulses. Optical damage threshold and two-photon absorption coefficient were determined for this pump pulses. Frequency conversion efficiency reached 0.24% at 1.85 mJ pump pulse energy and was decreased at higher one. Estimations indicate that application of 15 mm in diameter wide-aperture BaGa 2 GeSe 6 sample will allow one to increase pump pulse energy up to ~10 mJ, and to increase the mid-IR pulse energy up to 24 μJ at the same efficiency.

In order to increase the energy of a signal wave for a difference frequency generator, a spectral broadening of 100-fs pulses from a Ti:sapphire laser (center wavelength of 950 nm) is experimentally studied during their filamentation in five different gases: N2, NH3, He, CO2, and CO. The greatest broadening and the highest energy in the Stokes region of the spectrum are observed for CO2. Variation in the CO2 pressure in the gas cell shows that for a Ti:sapphire laser with a pulse energy of several mJ, the highest radiation energy in the Stokes wing with a shift of ∼1000 cm–1 is achieved at a pressure of 1 bar. Also, for each of the studied gases, the nonlinear refractive index is estimated at a pressure of 1 bar.

The simple mid-IR laser source based on femtosecond Ti:Sapphire laser and successive frequency conversion of its stretched up to 50 ps pulses in SrMoO4 Raman-active crystal and LiGaS2 second order nonlinear crystal is demonstrated. Energy of the mid-IR pulse at 11.4 mu m wavelength was up to 250 nJ that was four times higher than the previous results for the similar setup. The influence of pump pulse intensity on spectral and energetic parameters of laser pulses after SrMoO4 and LiGaS2 crystals were studied in details.

Filamentation of high-power femtosecond laser pulses in air is accompanied by a fairly strong release of optical energy into the propagation medium due to laser-induced ionization of air molecules and production of an underdense plasma of charged species. We present the results of our laboratory experiments and numerical simulations aimed at estimating the energy deposition amount by laser filament upon propagation in air depending on the conditions of spatial focusing, pulse energy, and radiation wavelength. Importantly, our study reveals a more than 50% decrease in the filament energy deposited in air in the range of moderate numerical aperture values, approximately from 0.003 to 0.007, at carrier wavelengths of 740 and 470 nm. We attribute such a considerable reduction in the laser pulse energy release for femtosecond plasma to the competing effects of Kerr self-focusing and geometric divergence of focused laser pulse.

Self-focusing and self-phase modulation of focused femtosecond laser pulse were studied in bulk fused silica. Depending on pulse energy magnitude, three different regimes were observed in the experiment: (1) linear regime, when the self-focusing effect is insignificant, characterized by linear spectral broadening; (2) transition regime, when the self-focusing effect notably reduces a laser beam spot in a focal plane without considerable nonlinear absorption and beam collapse, characterized by quick increase in spectral broadening due to self-phase modulation; (3) nonlinear regime, filamentation, characterized by near-to- linear spectral broadening due to self-phase modulation. The slope for the linear and nonlinear regimes was the same within the measurement error bars. The numerical simulation, using the ray transfer matrix method, envisioned the self-phase modulation spectral broadening effect in all these regimes.

We experimentally demonstrate laser system generating 100 -fs mid-IR pulses tunable within of 5.5-9.5 $\mu$m wavelength range. This system is based on difference frequency generation of femtosecond Ti:sapphire laser pulses in a nonlinear AgGaS 2 crystal.

The effect of focusing, taking into account self-focusing, on the interference of SRS (stimulated Raman scattering) and self-phase modulation in a 8-mm BaWO4 crystal pumped by laser pulses with a duration of 0.3 ps and a wavelength of 515 nm is experimentally studied. The maximum efficiency of SRS conversion (~23%) to the Stokes component of the ν1 = 925 cm–1 strongest mode is obtained with a lens with a focal length of 40 mm at the linear focus shift towards the rear facet of the crystal. The increase in efficiency, when the linear focus is shifted to the rear facet, is associated with an increase in the distance between the linear and nonlinear foci, which results in an increase in the effective length of the nonlinear interaction.

Transient stimulated Raman scattering (SRS) of chirped, temporally stretched up to 45 ps, Ti:sapphire laser pulses was studied in three different Raman-active crystals (BaWO4, SrMoO4 and Ca-3(VO4)(2)) at the same experimental conditions. BaWO4 and SrMoO4 crystals had the same SRS ''threshold'' energy/intensity, which was associated with their similar integrated cross sections for spontaneous Raman scattering. The highest energy SRS efficiency was obtained in SrMoO4 crystal and reached 8%, which was two-times higher than one in BaWO4. Higher SRS efficiency in SrMoO4 was observed due to lower nonlinear absorption. The Ca-3(VO4)(2) crystal had significantly higher SRS ''threshold'' and lower efficiency despite the lowest nonlinear absorption losses. A simple expression of exponential gain for transient SRS of chirped laser pulses was proposed and verified by comparison with experimental results. This expression was used to estimate dephasing time T-2 of Ca-3(VO4)(2) crystal which was 8.5 +/- 1.0 ps.

The prospects of SRS-active calcium/strontium orthovanadate crystals, Ca3(VO4)2 and Ca2.7Sr0.3(VO4)2, for spectral conversion of ultrashort visible-range laser pulses evaluated in a single-pass scheme with pump focusing into the samples. For the ~3 J incident laser pulses of 0.3 ps duration at wavelength of 515 nm the energy conversion efficiency of up to ~3.5% to the Stokes component shifted in frequency by ~850 cm–1 obtained in a 1.3 cm long sample of Ca2.7Sr0.3(VO4)2 crystal. Simultaneously, in the transmitted radiation spectrum the amplitude of Stokes component reached 1/3 of the amplitude at the pump wavelength. Under the same conditions, SRS in a Ca3(VO4)2 crystal was not detected at all. It was shown that the observed differences can be explained by the attenuation of pump pulse due to difference in two-photon absorption in these crystals.

Directivity patterns of terahertz emission by femtosecond laser filament plasma in air after radiation propagation in hollow tubes with metal and dielectric (polypropylene) walls are experimentally studied. It is shown that narrow directed radiation can be obtained using quasi-waveguide propagation in a tube with dielectric walls, while the terahertz pulse duration is not affected significantly.

Stimulated Raman scattering of chirped Ti:Sapphire laser pulses in BaWO4 crystal A spectrum transformation of a chirped Ti:sapphire laser pulse stretched to 0.2 ns (with initial transform limited pulse duration of 90 fs) was experimentally studied in a tandem of BaWO4 crystals. To increase stimulated Raman scattering efficiency, broadband laser emission was used as a seed of this process. Efficiency of the Stokes component generation corresponding to the BaWO4 crystal phonon mode ν1(Ag)≈925 cm-1 reached ~ 10%. Generation of the Stokes component corresponding to the weaker phonon mode ν3(Eg)≈795 cm-1, and second Stokes components of ν1(Ag) and ν3(Eg) modes were also observed. Mechanisms reducing the stimulated Raman scattering efficiency for the ν1(Ag) mode are discussed.

We study the propagation of ultrashort laser pulse in filamentation and postfilamentation regimes at the distances up to 95 m. In order to control the start of the filament and spectrum broadening we insert meshes inside the beam. For all beam configurations we found distances range where laser pulse triggers high-voltage discharge.

We performed full characterization of postfilament formed by the radiation of the Ti: Sa laser system on an extended atmospheric path. Single-shot angle-wavelength spectra, beam diameter and self-correlation function measurements have been employed for this purpose. Using angle-wavelength spectra, the evolution of on-axis red-shifted humps has been traced, showing that their divergence does not exceed 0.5 mrad. Two zones in the postfilamentation process have been revealed: the Stockes zone with soliton-like propagation, where the number of the Stockes humps and their shift increases while the pulse duration remains almost constant, and the zone where the temporal and spectral postfilament characteristics changes like in linear propagation mode, while the beam divergence is negligible due to the Kerr nonlinearity.