The pumping energy distribution across a ruby laser rod is one of the factors which may control the dominant mode of propagation through the laser assembly. The energy distribution is controlled by the focusing action of the rod and the absorption coefficient of the material. Experimental results are presented for a two-dimensional model, in which the energy distribution is shown to change from a central disc at low chromium concentrations to an annular ring, at about half the radius of the rod, for ruby rods that are about 1 cm in diameter and 0.05% chromium concentration. The results are in qualitative agreement with theory.
John G. Skinner, "Pumping Energy Distribution in Ruby Rods," Appl. Opt. 3, 963-965 (1964)