Thermo-optically perturbed TEM∞ oscillation in Nd: YAG lasers

Thermo-optic perturbations, i. e., depolarization and phase-front distortion, have been recognized as tha main limitation in achieving efficient TEM∞mode operation in cw or high pulse rate solid-state lasers. Previous investigations dealt with the principal nature of these effects by simply assuming uniform volumetric heating of the laser rod. In studying cw-pumped Nd:YAG systems, we found, however, that in order to relate the distrotion to the TEM∞performance, a more realistic thermal model must be adopted. Most significant is the radial fall-off in pump intensity and hence heat generation inside the cylindrical laser rod since (a) the depolarization depends strongly on the form of the thermal profile, and (b) a non-quadratic phase-front component is introduced. Analytical expressions for both types of resonator losses are presented and their effect on the TEM∞mode will be discussed. By employing a concave-convex resonator to obtain a large volume inside the active material, we observed that a critical mode diameter exists above which the increase in thermo-optic loss outweighs the gain in mode volume. For this optimum configuration, 2.5 watts of TEM∞power was obtained with an efficiency of 1/3 percent compared to 8 watts of multimode power from the same system. The three main loss components relating to scattering-absorption, depolarization and phase-front distortion were experimentally determined. Excellent agreement with the calculated loss values was observed. An evaluation of these findings aimed at an optimum TEM∞laser design will be presented.