Effect of pump-induced loss on the spectral dependence of Rhodamine 6G laser efficiency under microsecond coherent pumping

We examine the effect of the pump-induced loss in the gain medium of a Rhodamine 6G laser on the spectral dependence of the laser efficiency under microsecond coherent pumping. It is shown theoretically and experimentally that the pump-induced loss is mainly due to nonlinear refraction and thermo-optic inhomogeneities of the gain medium, which produce a negative thermal lens, whose defocusing effect reduces the laser efficiency. The induced absorption by excited-state singlet and triplet levels of dye molecules is shown to make no significant contribution to changes in laser efficiency. The pump-induced thermal loss at the peak absorption wavelength of Rhodamine 6G plays a more important role well above the threshold pump power density (γ ≥ 5). We demonstrate that the maximum dye laser efficiency under microsecond coherent pumping can only be achieved at an optimal pump wavelength.