Self-acousto-optic Raman-Nath modulation and nonorthogonal transversal modes are found in a broad-area Nd:YAG single-shot laser. The device is free from the thermal-induced effects previously related to orthogonality violation and the acousto-optic modulation comes from a shock wave produced by the discharge of the flash lamps that optically pump the laser. The experimental findings are reproduced by a general model of a class B laser.

A study on the spatio-temporal dynamics of broad-area Nd:YAG (yttrium aluminium garnet), Nd-doped phosphate glass and Nd-doped silicate glass lasers is presented to show the influence of the inhomogeneous gain profile and cross-relaxation phenomena on the spatio-temporal dynamics of the system. The suppression of the order-disorder transition shown in Cabrera et al (2006 Opt. Lett. 31 1067) for homogeneously broadened class B lasers is found for both glass lasers, independently of the strength of the cross-relaxation mechanisms. The results obtained indicate that a higher degree of inhomogeneous broadening leads to suppression of the filamentation in the transverse intensity pattern.

We report on the origin of an acousto-optic Raman-Nath self-modulation found in a broad-area Nd:YAG single-shot laser. Operating the laser device under vacuum conditions suppresses the spectral splitting associated with acousto-optic modulation by the shock waves produced by the discharge of the pumping flash lamps. This splitting is reproduced by a general class B laser model that takes into account the dynamical density grating generated by a stationary acoustic radial wave. The complex phenomena observed in high-power lasers form a vast and challenging field of research that has focused great attention during the last two decades. In class B solid-state lasers the nonlinear interaction of the laser field and the medium produces spatial and spectral hole burning, modal instabilities, restless optical vortex formation, beam filamentation, and many more complex phenomena [1][2][3]. Some of these effects reduce the spatial coherence, the energy yield or the general stability of the laser, and only a complete understanding of the dynamical processes involved can lead to the development of new laser devices free from these limitations. In other cases some of these phenomena can be exploited to create new applications or to achieve a better understanding of the nonlinear interactions that rule high-power laser spatiotemporal dynamics [4].In a previous work [5] we reported a splitting of the transverse mode-beating frequencies of a broad-area Nd:YAG single-shot laser. We explained that finding as the coupling between the laser field and a stationary acoustic wave transverse to the direction of propagation through the Raman-Nath effect [6,7]. The hypothesis developed there is that this acoustic stationary wave is mainly originated by the shock waves produced in the surrounding air by the operating flash lamps, but as long as we didn't have experimental evidence against other mechanisms we couldn't discard alternative explanations. In the present work we present experimental evidence of the disappearance of the spectral splitting when the laser device is operated under vacuum conditions, which proves that the acoustic wave detected in our laser rod has been transmitted through the air. Therefore, this effect should be detectable in other pulsed laser devices pumped by flash lamps. In particular those aimed to provide a high-power output, where very intense currents are reached inside the lamps, should develop the dynamics under study in the present work. Moreover, the implications of a mechanical wave altering the density and thus the refraction index of the gain medium should be studied, as this dynamical anisotropy could play an important role in many nonlinear phenomena, such as spatial hole burning, transverse mode locking, or even in pattern formation and beam filamentation.