Multi-transverse mode operation of alkali vapor lasers: modeling and comparison with experiments

In high-power diode pumped alkali lasers with stable resonators the radius of the pump beam is usually larger than that of the fundamental laser mode and thus several high order transverse modes of the resonator can participate in the lasing. A simple optical model of multi-transverse mode operation of alkali vapor lasers is reported. The model is based on calculations of the pump and laser beam intensities in the gain medium, where the laser beam intensity is a linear combination of the azimuthally-symmetric Laguerre-Gaussian modes. It was applied to Ti:Sapphire and diode pumped cesium vapor lasers. The model predicts that for low pump power only the fundamental lasing mode oscillates. However, for higher pump powers several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius were found. The model predicts linear dependence of the laser power on the pump power, the values of the former being in agreement with the experimental results obtained for diode pumped cesium laser [Electron. Lett.44, 582 (2008)]. The mode-matching efficiency for the multi-transverse mode lasing is ~0.8 - 0.85 which means that in this case almost complete overlap of the laser and pump beams takes place. The laser beam quality factorM²increases with increasing pump power from 1 at the threshold power to 5-6 at maximum values of the pump power resulting in lower beam quality at high powers.