RUNNING TRANSVERSE-WAVES IN OPTICAL-PHASE CONJUGATION

We investigate the dynamics of four-wave mixing processes in photorefr active crystals in paraxial approximation, assuming the phase-conjugat ion condition. By applying an external electric field to the crystal, we observe an onset of instabilities in the phase-conjugate beam, the generation of running transverse waves, their mutual collisions, and a continuous transition to a regime of spatiotemporal chaos. This state appears as irregularly oscillating defectlike patterns in one transve rse dimension. Running transverse waves are identified as the basic mo des of the system giving rise to a secondary instability. The observed regular and irregular spatiotemporal oscillations are characterized b y means of a spatiotemporal cross-correlation function and singular va lue decomposition. In the extension to two transverse dimensions the p ossibility of an even greater variety of regular and irregular pattern s is observed.