Abstract
Summary form only given. Shock waves have been extensively investigated in diverse areas of physics. However, their occurrence in optics is rather rare and thus far no such optical structures have been experimentally observed. In this paper we report a new family of kink-antikink shock waves that is possible in unbiased photorefactive media via the two wave mixing process. These waves move together as locked states, and have the interesting property of moving outside their initial ±? sector of propagation. The apparent direction of propagation and the spatial widths of these shock wave beams are directly related to their relative intensity. We provide a detailed analysis of these waves, and we illustrate their behavior with pertinent examples. © 2001 Optical Soc. Of America.

Abstract

Abstract
We report the first experimental observation of optical spatial shock-wave pairs. The shock waves consist of two coupled kink and antikink beams that remain locked to each other throughout propagation in a nonlinear diffusion-driven photorefractive crystal. These coupled shock-wave pairs move undistorted at angles that fall outside their original angular sector of propagation.

Abstract
We investigate the coherence characteristics of multimode incoherent spatial solitons in noninstantaneous Kerr-like nonlinear media. Other properties of these incoherent solitons are also discussed as a function of their modal composition. [S1063-651X(99)04001-5].

Abstract
We show that the vector beam evolution equations in properly oriented biased photorefractive media can exhibit bright-dark soliton pair solutions under steady-state conditions. These wave pairs are obtained perturbatively provided that the intensities of the two optical beams are approximately equal. Our analysis indicates that these bright-dark vector solitons exist irrespective of the polarity of the external bias field. The stability of these vector pairs has been investigated numerically and it has been found that they are stable only in the regime of positive bias polarity.

Abstract
The modulational instability of quasi-plane-wave optical beams in biased photorefractive media is investigated under steady-state conditions. The spatial. growth rate of the sideband perturbations is obtained by globally treating the space-charge field. Our analysis indicates that the growth rates depend on the strength of the externally applied electric field and, moreover, on the ratio of the optical beam's intensity to that of the dark irradiance. Our results are then compared to previous local treatments of the space-charge field equation. The two approaches are found to be in good agreement in the low spatial-frequency regime provided that the external bias field is sufficiently high. Conversely, in the high spatial-frequency region notable differences may exist. Relevant examples are provided.