218 Laser Physics, Vol. 8, No. 1, 1998, pp. 218–221. Original Text Copyright © 1998 by Astro, Ltd. English Translation Copyright © 1998 by åÄàä ç‡Û͇ /Interperiodica Publishing (Russia). 1. INTRODUCTION In [1], we experimentally investigated polarization characteristics of Nd, Cr : GSGG and Nd : YAG lasers with passive Q switching of the cavity by means of a Cr 4+ : YAG switch. It was demonstrated that, if the laser cavity includes a polarizer, the energy of the giant pulse and the polarization state of laser radiation depend on the angle of the passive Q switch (with respect to the cavity axis), the spatial arrangement of the switch rela- tive to the waist of the cavity, and the type of the polar- izer. It was shown that a cavity with a partial polarizer produces elliptically polarized radiation. The orienta- tion of the principal axis of the polarization ellipse (polarization azimuth) and the ellipticity degree of the laser beam produced by such a system depend on the angle of the Cr 4+ : YAG passive Q switch (with a period of 90°). These dependences were associated with self- induced anisotropy arising in the Cr 4+ : YAG passive Q switch at a certain stage of the development of a giant pulse when absorption saturation is switched on. The characteristic feature of this effect is that the azimuth of the polarization of radiation produced in such a system follows the orientation of phototropic Cr 4+ centers rela- tive to the plane of minimum linear cavity losses deter- mined by the orientation of the partial polarizer. The purpose of this study is to develop a model of a neodymium laser with a Cr 4+ : YAG passive Q switch and partial polarizer and to compare the predictions of this model with the experimental data. 2. THEORY In accordance with currently existing concepts, Cr 4+ phototropic centers in YAG crystals are considered as groups of linearly absorbing dipoles [2, 3] oriented along three orthogonal directions (along the principal crystallographic axes of YAG). As is well known [3–6], the propagation of high-power resonant radiation in a crystal medium with distinguished orientations of dipole moments of resonant transitions is accompanied by the appearance of self-induced anisotropy of satura- ble absorption. Specifically, this effect was observed in experiments on intracavity probing of a Cr 4+ : YAG crystal by high-power radiation with the wavelength λ = 1.06 μm [2, 6]. Self-induced anisotropy of satura- ble absorption should influence also (see [1]) the output characteristics (the lasing energy and polarization state) of a neodymium laser with a Cr 4+ : YAG passive Q switch when the switch is bleached (absorption is satu- rated) by a lasing pulse that passes many times through a medium of Cr 4+ phototropic centers. We will assume that the laser cavity includes an active element (an isotropic neodymium-containing material), a Cr 4+ : YAG passive Q switch, and a partial polarizer. Suppose that the optical axis of the cavity coincides with the orientation of one of three orthogo- nal groups of Cr 4+ centers (e.g., the [100]-axis, which perpendicular to the plane of Fig. 1). Then, the other two groups of Cr 4+ centers (the X s [010] and Y s [001] directions) lie in the plane of Fig. 1. Let the directions corresponding to minimum and maximum losses of the partial polarizer be defined as X p and Y p , respectively. Transmission in these directions will be denoted as T Xp PHYSICS OF SOLID STATE LASERS The Influence of Anisotropy of Nonlinear Absorption in a Cr 4+ : YAG Passive Q Switch upon the Polarization State of a Neodymium Laser N. N. Il’ichev, A. V. Kir’yanov, E. S. Gulyamova, and P. P. Pashinin General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 117942 Russia e-mail: ilichev@kapella.gpi.ru Received September 2, 1997 Abstract—The polarization state of a solid-state neodymium laser with a Cr 4+ : YAG passive Q switch and a partial polarizer is investigated. It is demonstrated that polarization properties of radiation are determined by the anisotropy of nonlinear absorption induced in the Cr 4+ : YAG passive Q switch. The results of calculations agree well with the experimental data. Y p Y s , [001] X s , [010] θ ϕ(t) X p E(t) Fig. 1. The scheme of calculation of the polarization state of a neodymium laser with a Cr 4+ : YAG passive Q switch.