TRANSVERSE MODULATIONAL INSTABILITY OF COUNTERPROPAGATING LIGHT WAVES

The interaction of two intense counterpropagating light waves in a finite, cubically nonlinear medium leads to a transverse modulational instability that results from the interaction of four electromagnetic sidebands. The convective gain spectrum and the absolute instability threshold of this instability are calculated analytically for two tractable limits and numerically for other cases. It is shown that phase conjugation, forward mixing, and Bragg reflection by four-wave mixing are three two-sideband limits of the four-sideband interaction. The four-sideband transverse modulational instability occurs in the spectral overlap region of these two-sideband limits and dominates the interaction of the two counterpropagating light waves over a significant range of the ratio of backward-to-forward pump intensities in both self-focusing and self-defocusing media. The absolute phase conjugation instability dominates only over a limited range of pump-intensity ratios in self-defocusing media.