Yttrium-aluminum garnet crystals with a gradient distribution of $\mathbf{Y b}^{3+}$ ions (Yb:YAG) were grown using the Czochralski method with liquid recharge. The concentration of $\mathbf{Y b}^{3+}$ ions along the length of the crystal boule varied from 0 to $4 \mathrm{at} . \%$. The subsequent studies showed the existence of a limiting gradient along the length of the crystals connected with formation of the impurities.

The method of laser scanning is proposed and developed for the investigation of radiation wavefront distortion in active elements of high-power laser amplifiers. Optical path difference profile is calculated from simultaneously investigated temperature and inverted population distributions in an active element. Optical path difference profiles introduced by pumped active elements of high-power laser amplifier are studied.

A novel method was proposed for the experimental investigation of wavefront distortion introduced to amplified radiation by pumped active elements in high-power laser amplifiers. The method is based on the simultaneous measurement of temperature distribution and the distribution of population density of the excited laser level in active elements. The underlying theory of the technique was presented; various factors affecting the accuracy of wavefront distortion determination were analyzed. The method was tested to study the wavefront distortion and the depolarization of radiation introduced by the Yb:YAG active element of a cryogenically cooled laser amplifier with high-power diode pumping. The focal length of the thermal lens was 0.40 ± 0.03 and 0.47 ± 0.05 m for the horizontal and vertical planes, respectively. The focal length of the electron lens was two orders of magnitude larger. The maximum value of losses induced by depolarization was 8.5%.

The numerical model of the laser amplification process based on the nonlinear Schrödinger equation for systems with simultaneously high peak power and pulse repetition rate has been developed. A model describes the interaction of the amplified radiation and the active element in the frequency domain without loss of phase relationships between the corresponding spectral components. The model is verified by comparing experimental and simulated results.

A simple and reliable approach for determining initial parameters in the problem of resonance Lorentzian curve approximation has been suggested. The algorithm has been applied to experimental data and its validity is shown. The results of iterative approximation with initial parameters that are found using proposed algorithm have been compared with those of approximation by the well-known Circle Fit algorithm. The suggested approach makes it possible to reduce signal-to-noise ratio requirements without loss of approximation reliability.

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 work is aimed at the investigation of the influence of nonlinear active ions concentration profiles in Yb:YAG laser elements on temperature distribution and wavefront distortions during amplification using sub-kilowatt level diode pumping. A mathematical model is presented for the theoretical study of the amplification process in crystals with cubic crystal system. A detailed comparison of Yb:YAG active elements with the same thickness and absorbed pumping power, but with various concentration profiles of Yb3+, ions is carried out. It is shown that the use of active elements with an increasing dopant concentration in the pump beam direction allows one to optimize the temperature profile inside the active element and, thus, reduce the thermal-induced wavefront distortions of the amplified radiation. Modeling is carried out for the experimentally grown crystal with linear concentration gradient profile. It is shown that the linear doping profile with a gradient of 0.65 at.%/mm allows increasing the small-signal gain up to 10% and decreasing the thermal-induced wavefront distortions by ~15%.

An approach associated with the use of active media with a non-uniform dopant ions concentration distribution is proposed to increase the pulse energy level and optimize the high power laser amplifier parameters. The optimal profiles of the laser-active ions gradient distribution in the amplifying medium were determined. A crystal sample was obtained in which the concentration of the activator changes sharply from 5 at.% to 10 at.% over a length of 3–5 mm.

This work is devoted to the method for studying temperature distribution in solid-state active elements of high average power laser amplifiers. Temperature distribution in an active element of the cryogenically-cooled diode-pumped laser amplifier with high average power were investigated and analyzed. The investigation was carried out for various pump beam diameters and repetition rates 500 and 1000 Hz.

A time-dependent model of laser amplification in an Yb:YAG crystal is developed, with accounting of active media thermophysical and laser characteristics dependencies on the temperature distribution, the dependence of the gain medium laser characteristics on the seed wavelength and the effect of amplified spontaneous emission. The numerical simulation of the laser amplification process in the active element of a multidisk amplifier was performed and compared to experimental results.

Currently, laser systems based on active elements doped with Yb3+ with simultaneously high pulse repetition rates and high peak power are in demand for many applications. High thermal load of active elements is the primary limiting factor for average power scaling. Experimental investigation of temperature distribution in active elements is of particular importance for estimation of cooling efficiency and for thermal processes’ monitoring. In the present work, the method of dynamic laser thermometry is proposed for temperature distribution investigation within cryogenically cooled Yb3+-doped active elements. The method is based on the dependence of the Yb3+ ion absorption cross-section on temperature at a wavelength of 1030 nm. The method was tested to study the 2D temperature map of the Yb:YAG active element of the high-power, diode-pumped, cryogenically cooled laser amplifier. The best measurement accuracy ±3 K is achieved at the maximal temperature 176 K. The results of numerical simulation are in good agreement with the experimental data. On the basis of the investigation, the quality of the cooling system is evaluated. The advantages and other possible applications of the method are discussed.

A time-dependent model of laser amplification in an Yb : YAG crystal is considered based on a system of balance equations, as well as radiation transfer and heat conduction equations. The model also takes into account the dependence of the laser characteristics of the gain medium on the injection wavelength and the effect of amplified spontaneous emission. This model is verified based on a diode-pumped amplifier with cryogenic cooling of active elements. The dependences of the gain on the pump pulse energy are experimentally measured for different amplification regimes and compared with simulation results.

The influence of optical centers concentration profiles on thermal fields under high-power end-face diode pumping is analyzed using numerical model. It is shown that non-uniform distribution profiles of optical centers concentration are very promising for high average pump power laser amplifiers. The possibility of creating concentration profiles of optical centers in gradient-activated crystals with YAG matrix with a specified distribution along the growth axis of the crystal is demonstrated. A technology of growing gradient-activated single crystal involving active mirror as active element is proposed and discussed.

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.

in this paper we focus on investigation of temperature distributions in laser elements based on Yb:YAG crystal at cryogenic temperatures. We propose the approach to enhancing of gain coefficient by decreasing of thermal load in active elements based on special concentration doping profile. The results of modeling are presented and discussed.

A numerical three-dimensional non-stationary model describing laser amplification process with account of the dependencies of properties of active element on temperature distribution was developed. A numerical simulation of the laser amplification process in cryogenically cooled high peak, high average power amplifier was performed and it is shown that optimization of pump parameters allows one to obtain up to 50% more energy at the output of the amplifier.

A time-dependent three-dimensional model for the laser amplification process has been constructed with allowance for the effect of the temperature distribution on the thermophysical and lasing characteristics of gain media. We have performed numerical modelling of the laser amplification process in the gain elements of a two-stage subjoule-level cryogenic laser amplifier operating at a pulse repetition rate of up to 1 kHz. It has been shown that taking into account the temperature distribution is of critical importance in calculation of cryogenically cooled laser amplifiers pumped with high-power diodes. We have found optimal diode pump parameters at which the maximum achievable pulse energy at the amplifier out-put can reach 300 and 570 mJ at pulse repetition rates of 1000 and 500 Hz, respectively.

The dynamics of phase distortions during laser amplification in the cryogenically cooled active elements with high power diode pump was modeled and analyzed. The possibility of significant reduction of wavefront distortions with small decrease of the gain coefficient by choosing the optimal values of the pump radiation radius is shown.

A high peak and high average power femtosecond laser system based on media doped with Yb3+ ions is being developed at the Institute of Laser Physics of the SB RAS. For efficient laser amplification and to avoid optical damage is actually to compensate wave front distortion caused by grating astigmatism in pump channel. Based on theory of propagation of gaussian beam in space and through optical elements the calculation of optimal parameters of two lenses telescope and comparison with experimental data has been performed. The obtained results can be used for decrease of astigmatic effect on beam profile quality in design of laser systems with elements involving astigmatism.

In this paper we focus on investigation of temperature distributions in laser elements based on Yb:YAG crystal at cryogenic temperatures. We consider the approach of using elements with non-uniform concentration distribution of dopant ions to reduce an average temperature and to minimize temperature gradients in active elements of laser amplifiers. The results of modeling are presented and discussed.