Abstract

Broadband spatially incoherent laser beams are being used to produce time- dependent focal spot structure in inertial confinement fusion (IGF) targets.^1,2 When averaged over time such beams give smooth irradiance profiles on target which can be useful in suppressing both hydrodynamic and plasma instabilities. A potential problem in the implementation of such beam-smoothing techniques on high-power Nd:glass lasers is, however, the enhancement of nonlinear self- focusing in the laser system for broadband compared to single-frequency (transform-limited) laser pulses. Since the broadband pulses of interest have excess bandwidth compared to their pulselength, they can have intensity peaks in time which are significantly larger than the average intensity. Because the nonlinear index of refraction in laser glasses is mainly electronic in origin and thus has a very fast time response, one would expect these intensity peaks to cause instantaneous self-focusing causing the beam to break up in space as seeded by perturbations present on the beam. However, since this enhanced self-focusing occurs over very limited periods of time, it is not at all obvious whether the time-integrated effect of such processes, or the optical damage it might produce, is any different from that for a temporally smooth beam of the same average intensity.

© 1992 Optical Society of America