Spatio-temporal dynamics in photorefractive two-wave mixing configurations: the counterpropagating geometry and the unidirectional ring oscillator

We investigate spatio-temporal dynamics of two-wave mixing processes in a p hotorefractive medium, considering two different configurations. In the cou nterpropagating geometry, above a primary instability threshold we observe the onset of spontaneous spatial modulation of the Gaussian beam profile du e to a static external electric field in the case of a self-focusing non-li nearity. Running transverse waves in the optical near field and wandering s pots in the Far field occur when one transverse dimension is taken into acc ount. Modulational inhomogeneities in the refractive index grating along th e transverse direction result in beam focusing effects. In the copropagatin g wave mixing geometry within a unidirectional ring resonator we find four different mode-like structures with either symmetric or antisymmetric field distributions. Regular and irregular spatio-temporal oscillations arise fo r certain values of the resonator detuning. The temporal evolution of the c haotic mode oscillation is represented by a heteroclinic tangency and the s patio-temporal oscillation is characterized by crisis-induced intermittency . (C) 1999 Elsevier Science Ltd. All rights reserved.