0021-3640/05/8103- $26.00 © 2005 Pleiades Publishing, Inc. 0090 JETP Letters, Vol. 81, No. 3, 2005, pp. 90–94. Translated from Pis’ma v Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, Vol. 81, No. 3, 2005, pp. 120–124. Original Russian Text Copyright © 2005 by Mukhin, Palashov, Khazanov, Ivanov. The creation of simple and reliable sub-megawatt lasers with diffraction divergence is one of the most interesting physical tasks for the next decade. A recent increase in the average power of solid-state lasers was so large that they can now compete with chemical and molecular lasers, which are the traditional leaders in power. Rapid advances in single-mode solid-state lasers make it possible to expect that the 100-kW limit will soon be overcome, and their small sizes and reli- ability ensure a number of new scientific, technologi- cal, and special applications. The inevitable heat release in the active elements is one of the fundamental problems limiting the power of a laser. This release leads to a photoelastic-effect-induced change in radia- tion polarization from point to point of the cross sec- tion, i.e., to the thermally induced depolarization of radiation [1, 2]. Thermally induced depolarization in cubic crystals with the [111] orientation was investigated in [3–6]. The problem of the effect of the crystal orientation and choice of the best orientation (the orientation for which depolarization is minimal) was formulated as early as in 1971 [7]. However, the results obtained by Koechner and Rice [7] are correct only for the [111] orientation, because they made a mistake that was pointed out in [1, 8]. Koechner and Rice [7] supposed that the directions of eigen polarizations coincided with the radial and tan- gential directions. The same erroneous statement was present in a classical book [2] that has gone through five editions. This circumstance required the development of a new approach to the theoretical analysis of this problem. This new approach is reported in this work. In the general form, we have analytically solved the clas- sical problem of thermally induced depolarization for an arbitrary orientation of any cubic crystal. A number of theorems on the physical distinguishability of the [001], [111], and [110] orientations have been proved, and the problem of the best and worst orientations has been solved. We present the experimental results for the thermal self-action of laser radiation that corroborate the theoretical conclusions. Influence of the Orientation of a Crystal on Thermal Polarization Effects in High-Power Solid-State Lasers I. B. Mukhin 1 , O. V. Palashov 1 , E. A. Khazanov 1 , and I. A. Ivanov 2 1 Institute of Applied Physics, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhni Novgorod, 603950 Russia e-mail: khazanov@appl.sci-nnov.ru 2 Institute of Materials Science, Zelenograd, Moscow region, 103460 Russia Received November 5, 2004; in final form, December 14, 2004 The polarization thermal self-action of laser radiation propagating in an isotropic crystal has been studied experimentally. New nonlinear effects have been observed for the first time. These effects include a change in the symmetry of the depolarized-field distribution and the qualitative dependence of the degree of the self- induced depolarization on the geometry of the beam and crystal. The classical problem of self-induced depo- larization has been analytically solved in the general form for an arbitrary orientation of any cubic crystal. The theoretical results agree well with the experimental data. The optimum orientation of the laser crystal has been determined, which can be effectively used in lasers with high average power. © 2005 Pleiades Publishing, Inc. PACS numbers: 42.60.Da, 42.25.Lc Fig. 1. Cross section of the cylindrical active element, where e 1 and e 2 are the eigen polarizations at the point (r, ϕ) and E is the radiation polarization.