A comparative analysis of various focusing schemes for few-cycle femtosecond pulses in collinear and non-collinear geometries is carried out. Focusing with a parabolic mirror is considered, as well as spherical (mirrors are located on a sphere, beams are focused to the center), ring (beams in one plane are focused to the center), 2 row ring geometries. It is shown that in the noncollinear scheme there is an optimal number of beams, depending on the chosen geometry, at which the maximum intensity is achieved. The spatial contrast and the space-time distribution in a focused beam with coherent combining are analyzed. The Monte Carlo method is used to study the influence of the parameters of the beams being combined, such as aberrations and angular instability on the efficiency of coherent combining in collinear and noncollinear geometries.
The results of experimental and theoretical studies of behavior spectral profile of second harmonic (SH) radiation with a central wavelength of 475-477 nm during its formation in a nonlinear KDP crystal, depending on the phase matching angle and radiation intensity are presented. It is shown that a change in the propagation angle of fundamental radiation to one or the other side of phase matching angle by 16– 20 min leads to a spatially inhomogeneous broadening of the second harmonic spectrum up to two times and a shift of its maximum (averaged over the beam cross section) to shorter wavelengths from the central wavelength.
The paper presents analysis results of large aperture stretcher schemes for use in laser systems after pre-amplification stages. To increase output peak power in the two-channel femtosecond laser system with coherent field combining, developed at ILP SB RAS, a stretcher for deployment in front of additional amplification stages is required. The stretcher and additional stages will boost pulse energy in a channel up to 1.5 J. Since beam energy at stretcher input is 100-150 mJ, beam diameter exceeds 20 mm what, is determined by damage thresholds of stretcher elements. With a stretching ratio of 105 and such an aperture, aberrational distortions of the stretcher obstruct further compression, focusing, and effective coherent combining of amplified pulses. Optimized parameters of the off-axis and off-plane Offner stretchers allow pulse stretching to 2.1 ns (spectrally limited duration ~20 fs and carrier wavelength 0.83 μm) and subsequent recompression down to 24 fs and 35 fs in a beam with 20 mm aperture.
Features of compression of large-aperture negatively chirped amplified pulses of various aperture and durations up to a few-cycle in IR spectral regions are analyzed. Using the modified ray-tracing algorithm, the spectral-angular dependences of the radiation at different points of the beam cross section at the outputs of the grating compressors constructed according to the Offner, Martinez scheme and the scheme with a single-lens internal telescope are calculated. Offner compressor scheme has minimal aberrations and introduces a spectral angular chirp with a divergence much lower than the beam diffraction divergence. It was shown that in the region of 1.4 μm, negatively chirped pulses can be compressed down to near bandwidth limited duration of ~30 fs over the entire 40 mm beam aperture. It was also shown that, using stretchers with an internal telescope based on the Offner scheme, in the region of 3.6 μm it is possible to compress amplified negatively chirped pulses with duration of 40 ps and with a spectrum width close to an octave down to near bandwidth-limited duration of 18 fs duration at a 20 mm aperture.
Scheme for pulse compression during self-phase modulation in nonlinear media with a different sign of effective cubic nonlinearity is presented and investigated. It is shown that the proposed scheme is the most promising for implementing the regime of compression of powerful pulses to few-cycle ones in the spectral range of 1.55. In this case the maximum energy of compressed pulses is limited by the available aperture of nonlinear optical crystals. In the region of 1.55 μm, when using 5 mm BBO crystals, the achievable duration of compressed pulses is found to be 7.4 fs. The magnitude of the final nonlinear phase shift in the range of compressed beam intensities of 75-100% is more than an order of magnitude lower compared to multi-element spectral broadening schemes based on media with positive cubic nonlinearity. The proposed schemes can also be used to compress pulses in other spectral ranges with an appropriate selection of nonlinear optical crystals.
The possibilities of realization of the output amplifier cascade of a powerful laser system based on parametric amplification of femtosecond pulses with multiple beams pumping are investigated. A method for choosing the optimal arrangement of the pump beams of a parametric amplifier which makes it possible to achieve a broad spectral gain band with minimal losses from parametric diffraction is proposed. A number of optimal arrangements of the pump beams in LBO and DKDP crystals are chosen. Simulation results of the parametric amplification taking parametric diffraction into account are presented for these crystals. The dependence of the amplification efficiency and the amplified pulse duration on the number of pump beams is analyzed. Complete elimination of modulation of the spatial profile of the amplified radiation is demonstrated with the use of spatial filters in case of LBO crystal. The influence of small-scale self-focusing of the interference pattern of the total pump beam on the peak intensity of beams is investigated for LBO and DKDP crystals. It is shown that DKDP crystals are of little avail for multiple pump parametric amplification of fs pulses in 700- 900 nm spectral range.
The stabilization system implemented has allowed one to achieve phase residual instability ~0.17 radian (rms) for the 30 fs-pulse, which is sufficient for nonlinear interaction radiation with optical medium in forthcoming lightwave electronics experiments.
Recent theoretical and experimental studies show that filamentation of femtosecond pulses in middle infrared range in solids can generate intense supercontinuum (SC) with width of a several octaves. The structure and dynamics of the spectrum formation is different from those observed during filamentation of femtosecond pulses in the near infrared range of the spectrum. With numerical modeling we investigate features of infrared (IR) SC generation in a number of solid-state media transparent in a broad infrared range with various bandgaps, including KBr (7.6 eV), ZnS (3.68 eV) and ZnSe (2.71 eV) for different pump wavelengths. SC formation dynamics is analyzed. With comparison of different media, including one with artificial bandgap, we find that lower bandgap decreases SC bandwidth and filamentation length.
Nonlinear optical schemes for generation of terawatt femtosecond pulses in the range of 2-10 μm using multiterawatt laser radiation of the two-channel system created in the ILP SB RAS are presented and discussed. It is shown that the use of large aperture LBO crystals and AgGaGeS4 for difference frequency generation and parametric amplification enables to generate a wideband radiation in the range of 2-10 μm.
Comparative analysis of optimal scheme of non-collinear optical chirped-pulse parametric amplification of fewcycle
femtosecond pulses from Ti:Sa laser in LBO and DKDP crystals pumped by picosecond pulses up to petawatt level
is presented. A flexible code, based on the extended model of parametric amplification, which takes into account the
large set of effects such as saturation, phase self-modulation, influence of beam divergence, thermal effects, and
amplification of spontaneous emission was realized. A way of creating nearly 1 PW system based on LBO crystals with
transform-limited pulse duration about 9 fs has been demonstrated. Comparison between DKDP and LBO crystal
showed that the latter is much better for OPCPA petawatt system design than DKDP.
The results of theoretical and experimental study of thermal lensing in diode-pumped Yb:YVO4 laser crystal,
Yb:Y2O3 and Yb:Sc2O3 laser ceramics are presented. Shown, that influence of thermo-lensing effect is necessary to
consider for creation of effective high-intensity femtosecond Yb-doped laser systems.
In this work we have performed the experimental researches of features for the generation of supercontinuum in laser materials with identical chemical composition: Yb:YAG crystal and Yb:YAG laser nanoceramics. Dependence of width of supercontinuum spectrum in 515-1100 nm spectral range on femtosecond radiation intensity was investigated. At laser intensity ~1.2•1014 W/cm2 the short-wave wing of a spectrum for nanoceramics has greater intensity and more
flat shape in comparison with crystal. Experiments were made at lens focusing of the Ti:Sapphire femtosecond laser system radiation with energy up to 0.5 mJ in explored sample that was inside of integrating optical sphere. Also we investigated the interaction of femtosecond laser pulses and the generation of supercontinuum in Nd:Y2O3 nanoceramics. The maximum value of laser intensity in experiments was restricted by optical breakdown on target output surface, i.e. was below threshold of ablation of sample substance.
The results of experimental spectroscopic and laser investigations of ytterbium-doped partially (mixed) disordered
tetragonal gadolinium-yttrium vanadate crystals at 5-300 K temperature range under laser diode pumping are presented
and compared with yttrium vanadate crystals. The Yb:GdxY1-xVO4 (x=0.64) partially (mixed) disordered crystals
demonstrate large stimulated emission cross-section compared with the values of Yb:YVO4 crystals. Polarized
absorption and fluorescence spectra in the 5-300 K temperature range are investigated and gain cross-sections are
deduced at room and liquid nitrogen temperatures. The amplitudes of emission cross-sections are increased almost
fourfold at liquid nitrogen temperature in comparison with the same at room temperature at wavelength near 1006 nm.
The CW and ultrashort pulses generation with additional SESAM structure have been investigated in the folded
resonator at 975 nm laser diode pumping at room temperature. At low temperature the estimations for different inversion
population ratios show that duration of pulses are somewhat varied.
The elastic and thermo-optical properties of chrysoberyl, beryllium hexaaluminate and beryllium-lanthanum hexaaluminate crystals have been experimentally studied. The velocities of elastic-wave propagation in the crystals are measured by acousto-optic interference method. The values of all the independent components of elastic-constant tensor are determined and used to calculate a number of important dynamic parameters of the crystals such as the Young's and shear moduli, the modulus of volume elasticity, Poisson's ratio, the Debye temperature. Also measurements of refractive
indices in 25 - 75 C temperature range in VIS spectral region were performed. Using experimental data the dispersion of
thermal optical coefficients (dn/dT) was calculated, these data were employed to evaluate the thermal lens in beryllium
containing laser crystals. The experimental and calculated data are compared with similar parameters for well-known
laser hosts. Some of beryllium containing oxide crystals was shown to be candidates for master oscillator and amplifying
stages of high power femtosecond laser systems.
Experimental results of self-compression of femtosecond pulses under filamentation in argon and xenon are
presented. The mode of a self-compression in xenon is realized for the first time. The dependence of the spectrum
broadening from pressure of these gases, input energy and focusing parameters are studied in detail. The spectral and
temporary profiles of the first and the second filaments at multiple filamentation are analyzed. Features of multiple
filamentation are revealed in xenon. For the first time experimentally the effect of restriction a number of filaments and
effective swapping of energy from one filament to another (more than 70 % of energy in two-filament mode without
increasing of their amount is founded). The possible mechanism of the phenomenon related with the saturation of the
third order nonlinearity in xenon and influence of the higher fifth-order susceptibility χ(5) are discussed.
Hybrid laser medium on the base of the Yb:YVO4 and Yb:YAG (and Yb:KYW) crystals with overlapping broadband
gain contours in common cavity was experimentally analyzed. On the absorption and fluorescence data at liquid helium
temperature, excited states lifetimes the energy of electronic levels of ytterbium in vanadate crystalline hosts were
calculated and the lasing parameters of broadband transitions of doped ions were studied. It has been shown that, this
way of forming of the stationary regime of ultrashort pulses generation with duration in accordance with combined gain
bandwidth in the laser with hybrid active medium is perspective.
In this work the opportunity of realization of laser action on vibronic transitions of Cr3+ ions in new crystal - hexaaluminate of beryllium-lanthanum (BeLaAl11019:Cr) is investigated. The thermodynamic and physical parameters of host crystal were studied. The experimental spectroscopic and relaxation properties of Cr3+ ions have been performed. Absorption and fluorescence spectra are characteristic for octahedral coordinated trivalent ions. The emission cross-section of broadband 4T2-4A2 transition is determined (~2•1O-20CM2). In configuration curves model the basic features of fluorescence and perspective of lasing in the 700-1000 nm range are considered. The new laser crystals BeLaAl11O19:Nd were grown by the Czochralski methods. This material has broad absorption bands at 580, 740 and 790 nm, the latest can be used for LD pumping. The broadest emission lines at 1050 and 1080 nm are perspective for ultrashort laser pulses generation. The intensity parameters, spontaneous emission probabilities, the inter-manifold branching ratios and fluorescent lifetime have been calculated by means of Judd-Ofelt theory and compared with the experiment. The CW generation was realized under Ar-laser pump and laser properties were investigated. The investigation shows that the BeLaAl11O19 is a promising host for a creature the new solid state laser media.
Design of a high-power femtosecond laser system based on hybrid chirped pulse amplification (CPA) technique developed by us is presented. The goal of the hybrid principle is the use of the parametric and laser amplification methods in chirped pulse amplifiers. It makes it possible to amplify the low-cycle pulses with a duration of ≤ fs to terawatt power with a high contrast and high conversion efficiency of the pump radiation. In a created system the Ti:Sapphire laser with 10 fs pulses at 810 nm and output energy about 1-3 nJ will be used like seed source. The oscillator pulses were stretched to duration of about 500 ps by an all-reflective grating stretcher. Then the stretched pulses are injected into a nondegenerate noncollinear optical parametric amplifier (NOPA) on the two BBO crystals. After amplification in NOPA the residual pump was used in a bow-tie four pass amplifier with hybrid active medium (based on Al203:Ti3+ and BeAl2O4:Ti3+ crystals). The final stage of the amplification system consists of two channels, namely NIR (820 nm) and short-VIS (410 nm). Numerical simulation has shown that the terawatt level of output power can be achieved also in a short-VIS channel at the pumping of the double-crystal BBO NOPA by the radiation of the fourth harmonic of the Nd:YAG laser at 266 nm. Experimentally parametric amplification in BBO crystals of 30-50 fs
pulses were investigated and optimized using SPIDER technique and single-shot autocomelator for the realization of shortest duration 40 fs.
New Yb:YVO4 and Yb:GdxY1-xVO4 (x=0.6) laser crystals were grown by the Czochralski methods. The experimental spectroscopic properties of trivalent ytterbium ions in tetragonal vanadates have been performed. Polarized absorption and fluorescence spectra were investigated and stimulated emission cross sections were estimated for both crystals at room (RT) and liquid nitrogen (LN) temperatures. On the absorption and fluorescence data, excited states lifetimes the energy of electronic levels of ytterbium in crystalline hosts were calculated and the lasing parameters of broadband transitions of doped ions were studied. The investigation shows these crystals are perspective for femtosecond pulses generation in NIR region near 1000 nm under laser diode pumping.
The new laser crystals BeAl6O10:Cr3+ were grown, spectral-luminescence and CW laser properties were investigated and compared with those of well-known laser medium-alexandrite (BeAl2O4:Cr3+). CW laser generation on vibronic transition 4T2-4A2 of Cr3+ ions in BeAl6O10 crystals was realized in the range of 800-880 nm under Ar+ laser pumping. The emission cross-section of laser transition was estimated about 6×10-20 cm2. We confirmed these crystals are perspective for generation of femtosecond pulses in the near IR region under LD pumping.
Femtosecond pulse generation in Al2O3:Ti3+ laser with some types of laser cavity configuration with semiconductor saturable absorber mirror (SESAM), based on semiconductor quantum well low temperature (LT) GaAs/AlAs, GaxIn1-xAs/AlyGa1-yAs saturated absorbers and metal mirrors have been investigated.
Design of novel integrated structure-semiconductor broadband saturable-absorber dispersion controlled mirrors for mid IR spectral range lasers have bene discussed. The novel design of SESAM is based on the multilayer structure like Gires- Tournios interferometer. High reflector consists of the metal mirror with low losses in this spectral range. Top section consists a few layers forming the linear grown dependence of group delay by resonance effects of interferometer. The thin HgCdTe quantum well layer with nonlinear absorption can be included in transparent semiconductor layer near the antinode of standing wave. This structure has low amounts of nonequal thickness layers that create the high reproducibility way of such type structure experimental realization. SESAM design for Cr:ZnSe laser is analyzed in detail.
Numerical simulation of ultrashort pulse generation in the laser with a composite active medium and additional Raman active element in a cavity has been done. It was created that for some laser parameters the optimization of a Raman gain and a frequency shift values was resulted in additional shortening of pulse duration.
The new laser crystals BeLaAl11O19 doped with Cr3+, Ti3+ and Nd3+ ions were grown by the Czochralski technique. The absorption and fluorescence spectra of impurity ions are reported and the temperature dependence of the fluorescence lifetime are described. The laser properties of these ions were investigated. The laser action has been achieved on 4F3/2-4I11/2 (1052 nm) transition of Nd3+ -ions under selective laser pumping. The physical properties of BeLaAl11O19 crystal were studied: the values of all independent component of elastic constant tensor were determined. On the base of a number of dynamic parameters of crystals, such as Young's modulus, the shear modulus, the volume elasticity modulus and Poisson's factor, Debye temperature and specific heat capacity were calculated. The investigation show that the BeLaAl11O19 is a promising host for a creature the new solid state laser media.
One of the possible methods of realization of active medium with anomalously wide bandwidth has been described. Composite active medium, consisting of several laser active centers with overlapping gainbands in common resonator has been created. In this case gain contour has complex shape with local extremums. Method of numerical simulation of the formation dynamics of ultrashort pulse at passive mode locking in laser with arbitrary spectral gain contour has been performed. The main parameters for the generation of ultrashort pulse in a laser with a composite active medium are obtained and investigated. The conditions of realization of stationary regime in the form of ultrashort pulse generation with duration determined by combined gain bandwidth are calculated.
It has been proposed the design of mirrors for near IR femtosecond lasers with given pulse characteristics based on the III-V compound semiconductors using MBE-technologies with reproducible phase parameter due to precise control of the layer parameters. The designed structure of two-part multilayer mirror includes the bottom part with a great number of AlGaAs-pairs and the top several antireflection layers. The specified negative near-constant group delay dispersion can be realized at certain band of spectrum.
New active media for near IR cw tunable lasers based on F2+O2- color centers in additively colored crystals NaCl, KCl, KBr, RbCl doped with hydroxide ions OH- were studied. The optimal conditions of these centers formation were determined. The generation characteristics of cw lasers based on F2+-centers in LiF, TL0(I)-centers in KCl:TI, FA(II)-centers in KCl:Li, and F+O2--centers in crystals NaCl:OH, KCl:OH are presented.
Using the spectroscopic data obtained, the possibility of producing stimulated generation in doped solid state media -- beryllium aluminates doped by 3-D-ions (Ti3+, V4+, V3+, Ni2+, Cr3+) -- is considered and a series of emission parameters of Ti and Cr ions is presented.
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