Pump power dependence of ruby laser starting and stopping time

Experimental results from a number of small ruby lasers show a linear relationship between the pump power and the inverse of the time delay between pump flash and the onset of laser oscillation. These experimental results are quantitatively predicted by a simple rate-equation analysis, yielding a single theoretical curve with no adjustable parameters and in good agreement with the experiments. Additional evidence is also presented verifying the onset at higher pump powers of additional "bouncing-ball" or "light-pipe" modes of oscillation in ruby rods with polished side walls. These additional modes account, in particular, for the abrupt cessation of laser output after an anomalously short duration of laser action, as is typically observed in such rods. These anomalous modes, which are usually not well coupled to the detection circuitry, apparently suppress the normal mode of oscillation once they begin and limit the available output energy in the normal mode. The anomalous modes are eliminated by roughening the sides of the rod, after which normal duration of laser action is observed. The laser stopping time is also predicted with reasonable accuracy by the rate-equation analysis.