KEYWORDS: Laser radiation, Laser development, Temperature distribution, Optical path differences, High power lasers, Wavefront distortions, Cryogenics, Laser systems engineering
The work is devoted to the investigation of gain complex dependency on the pump radiation parameters. The dependencies of the pulse energy gain and the parameters of the amplified radiation on the parameters of the pump radiation in the amplifying cascade based on Yb:YAG active elements are experimentally studied. Small-signal gain with the magnitude of 19 was reached in three consecutive active elements with good beam quality and high pointing stability. A method is proposed for simultaneous experimental investigation of temperature and gain coefficient distributions. Thermal contribution to optical path difference is about 3 wavelengths of amplified radiation while the electronic contribution is 100 times smaller.
The numerical three-dimensional non-stationary model based on balance equations and the thermal conductivity equation was used to study the dynamic of laser amplification process in Yb:YAG media. Characteristic curves for gain and optical phase difference are obtained, and it is shown that it is possible to minimize wavefront aberrations in amplified radiation while keeping high gain coefficient by choosing optimal pump parameters. Transversal doping profiles are proposed for decreasing of losses arising from amplified stimulated emission.
This paper is focused on the investigation of the effect of temperature on the laser amplification process. In case of Ybdoped active media there is a significant dependency of laser and physical properties on temperature that leads to reducing of gain coefficient. Theoretical description and experimental study carried out based on Yb:YAG crystal pumped by high power laser diode. Despite the cooling of the active element, the heating significantly affects at cwpumping regime and determines the shape of gain coefficient dependency on the pump power.
To supress beam pointing instabilities after high power laser multipass amplifier with cryogenic closed-loop cooling the copper heatsinks were optimized. Active elements’ temperature dependency on diode pump regimes is measured. The alternative seed laser for the high power laser multipass amplifier with cryogenic closed-loop cooling was modeled and designed.
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.
A numerical simulation of the generation of supercontinuum (SC) in highly-nonlinear photonic crystal fibre pumped with picoseconds-long pulses has been done using the RK4IP algorithm. Simulation and experimental results are in good agreement. The supercontinuum will be used for further amplification.
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.
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.
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 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.
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.
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.
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.
The possibility of creation of self-frequency doubling laser crystal based on crystal KTP doped with chromium ions is investigated. The spectral properties of Cr3+ ions are researched. The magnitude of emission cross-section at the wavelength of 833 nm corresponding to the maximum of vibronic transition 4T2-4A2 of Cr3+ ion is estimated. The excited state lifetimes at room and liquid nitrogen temperatures are determined. The spectroscopic investigations show that impurity Cr3+ ions in KTP crystal are placed in positions with weak crystal field: Dq equals 1507 cm-1, B equals 652 cm-1, C equals 2800 cm-1. The configuration diagram model with one excited state is discussed. The main features of fluorescence spectra and the expected laser emission in the near IR range form 700 to 1000 nm are considered.
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.
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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