Optical ray retracing in stimulated Brillouin backscatter from laser-produced plasmas is studied numerically, using a 2-D propagation code (BOUNCE). This code treats steady-state behavior in the strong damping limit, and includes self-focusing effects. Ray retracing phenomena are grouped into two limiting cases. In the "whole-beam" limit, the pump field at the lens has a broad spatial profile that can be focused to a long narrow waist within the plasma. This geometry selectively amplifies only those initial noise components that are propagating back along the axis, where the net gain is highest. The simulations show that such effects exhibit a pronounced threshold due to self-focusing, and disappear when the focal width becomes comparable to the length of the gain medium. In the opposite limit, where the pump radiation at the lens is dominated by small-scale transverse structure, the backscatter can reproduce this structure in detail (i.e., exhibit wavefront reversal) when the far field of the pump produces an interference pattern in the plasma. The plasma then behaves as an active volume hologram as it amplifies the backward-propagating random noise fields. If the target is moved far out of focus so that the pump components can no longer interfere, the ray retracing disappears, and the actual backscatter profile depends upon the noise structure. Wavefront reversal is found to be influenced by several other factors, including focal spot size, spatial gain narrowing, gain inhomogeneity, and self-focusing. The simulations show that it is most pronounced when the spatial profiles of the pump and the backscatter are wide in comparison to the transverse wavelengths of the interference structure, and the $\frac{1}{e}$ gain length (especially in the last one or two $e$ foldings) is large in comparison to the optical diffraction lengths. It is enhanced significantly if the plasma has a monotonically decreasing gain coefficient away from the target. In the strong-damping limit, where the nonlinear phase change/cm is comparable to the gain coefficient, filamentation can be important near the target; however, its effect on small-scale ray retracing is usually minimal, provided that the above conditions are satisfied. If the focal spot decreases below a critical width, however, whole-beam self-focusing takes over and quickly destroys the small-scale retrace.

• Received 22 January 1980

DOI:https://doi.org/10.1103/PhysRevA.22.2156