Coupled Finite Element Analysis of Composite Laser Rods Thermal Characteristics under Longitudinal Diode Pumping

Diode laser pumped solid-state lasers are covering a wide range of applications. The longitudinal pumping geometry of laser rod is particularly favorable as it provides a high degree of spatial overlap between pump and lasing modes. However, high pump-power densities are required to achieve sufficient inversion in the laser material. This produces high thermal loading in laser crystals, which, in turn, leads to undesirable thermal effects, such as temperature-dependent index change, temperature-dependent stresses, and end-surface deformation. All those effects are qualified as thermal lensing in laser material.

Yttrium aluminum garnet (YAG) crystals doped with Nd, Er, Yb and other ions are currently the most popular laser crystals for diode laser pumped solid-state lasers, especially in end-pumped configurations, that is why laser rods with YAG host material are chosen as the objects of modelling. A multiphysics approach and finite-element method are used to the numerical simulation of thermal lensing in such rods. Thermo-mechanical behavior is considered by applying the ANSYS software, while particular pre-processor for generation of the heat source as well as postprocessor for evaluation of the optical path difference (OPD) is developed.

The difference between standard crystal blocks and composite structures with undoped end caps is investigated. The results show that OPD for one pass through the crystal in all cases is practically the same. Thus, the composite rod geometry does not reduce significantly thermal lensing, however temperature gradients are considerable smaller. The obtained results also show that the use of the composite AE practically does not decrease the thermally induced aberrations of spherical type. Therefore, the quality of the probe or laser beam will change mainly in the same way in both the conventional and the composite AE pumped with Gaussian beams.