Effects of nonlinear propagation on laser focusing properties

In a high-power laser system both the nonlinear growth of small-scale spatial irregularities on a pulse (beam breakup) and the accumulation of self-induced phase-front distortion on the spatial envelope of the pulse (whole-beam self-focusing) alter the focusing properties of the system output. We present experimental results which show simultaneously and in detail the influence of these two effects on the spatial and temporal profiles of the focused output of a simple system. in an extreme case the focused on-axis intensity is observed to be reduced by beam breakup to 20 percent of the value obtained in the absence of breakup. Diffraction code calculations of whole-beam self-focusing, which in these experiments tends to enhance the focused on-axis intensity, are found to agree closely with the measured results and demonstrate that whole-beam self-focusing effects depend strongly on the plane being observed.