Spontaneous pulsations and polarization multistability resulting from the nonlinear interaction of two standing waves with an active medium in a single-longitudinal-mode Fabry-Pérot gas laser with a weakly anisotropic cavity have been studied at different cavity anisotropies and different transitions between the working levels with and without an axial magnetic field applied to the active medium. As predicted theoretically, a series of polarization dynamical phenomena have been observed experimentally in the He-Ne (ϭ1.15 m͒ laser in the case of orthogonal elliptically polarized eigenstates of the cavity. Periodic oscillations of two qualitatively different forms ͑distorted sinusoids and large spikes͒, the transition between two stationary orthogonal elliptically polarized waves in the vicinity of the line center tuning which occurs through the region with polarization instability, as well as switches between the orthogonally polarized components when the output is nonstationary have been found. It is shown that the diversity of nonstationary polarization phenomena in gas lasers with weakly anisotropic cavities is due to spontaneous pulsations of intensities, ellipticities, and azimuths of two emitted waves, caused by the competition of the nonlinear active medium and empty cavity anisotropies. The results of the theoretical modeling are in quantitative agreement with the experimental results.
A comprehensive analysis of gened relativistic spacetimes which admit a shearfree, irrotational and geodesic time-like coogruence is presented. The equations governing the models for a gened energy-momentum tensor are wrilten down. Coordinates in which the metric of such spacetimes takes on a simplified form are established. The general subwses of 'zero anisotropic stress', 'zero heat-flux vector' and 'two-component Ruids' are investigated. In particular, perfect-fluid Friedmann-Robertson-Walker models and spatially homogeneous models are discussed. Models with a variety of physically relevant energy-momentum tensors are considered. Anisowopic fluid models and viscous fluid models with heat conduction are examined. Also. models with B perfect Ruid plus a magnetic field or with pure radiation. and models with two non-collinw perfect fluids (satisfying a variety of physical conditions) are inwtigated. In particular. models with a (single) perfect Ruid which is tilting with respect to the shem-free, vorticity-free and acceleration-free time-like congruence are discussed.PACS numbers: 0420J. 9880H
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