Abstract

We have observed that the polarizations and intensities of counterpropagating laser beams in sodium vapor are temporally unstable. The output from a stabilized dye laser tuned near to the 3S_1/2 → 3P_1/2 atomic transition was split to form the two counterpropagating beams. These beams were polarized with the same linear polarization and were weakly focused to a spot size of ~750 μm in sodium vapor having a density of ~3 × 10¹³ cm⁻³. We monitored the light generated in the polarization orthogonal to that of the pump beams. For a detuning of 1.01 GHz to the high-frequency side of the 3S_1/2(F = 2) → 3P_1/2(F = 2) transition and a power of 160 mW in one of the beams, we found that the threshold for the instability occurred at a power of 15 mW in the other beam. Slightly above threshold, the instability was found to be periodic in nature. As the intensity in the counterpropagating beam was increased far above the threshold value, wild fluctuations in the polarizations and intensities of the transmitted beams were observed. We have found that these fluctuations are chaotic and that the system evolves on a strange attractor whose dimension increases with increasing laser power. These observations are in qualitative agreement with the theoretical predictions of Gaeta et al.,¹ which predict that the polarizations of counterpropagating beams in a Kerr medium are unstable to the growth of temporal fluctuations.

© 1988 Optical Society of America