https://hal.archives-ouvertes.fr/hal-00140068/document

On thermal effects in solid state lasers: the case of ytterbium-doped materials

The optimum mode-to-pump ratio in scaling fiber-coupled laser-diode end-pumped lasers to higher power has been investigated by including the thermal effect into the space-dependent rate equation analysis. The optical path difference (OPD) distribution has been derived as a function of the pump-beam quality, focus position of pumping light, and pump radius at the focal plane under the assumption that the end faces of the crystal are thermally insulated. The diffraction losses arising from thermally induced spherical aberration have been estimated by the Strehl intensity ratio. The present results for the optimum mode-to-pump ratio are markedly different from previous analyses in which thermal effects are neglected. Here, the optimum mode-to-pump ratio is a decreasing function of input pump power, and is less than unity in the case of a slightly high pump power. The practical example of a Nd:YAG laser pumped by a 13-W fiber-coupled laser diode is considered to confirm our physical analysis.

Optimization in scaling fiber-coupled laser-diode end-pumped lasers to higher power: influence of thermal effect

Degradation in beam quality due to aberrated thermal lensing and depolarization loss due to stress induced birefringence in end-pumped edge-cooled solid-state lasers are investigated. A simple model for the dependence of thermal lensing and degradation in beam quality on the transverse intensity profile of the pump beam is presented. Experimental measurements of thermal lensing, degradation in beam quality and depolarization loss in a diode-bar-end-pumped Nd:YAG rod support the main predictions of the model and, in addition, show that there can be significant extra heating under non-lasing conditions compared to lasing conditions. The role of energy transfer upconversion as a mechanism for additional heat loading under non-lasing conditions is considered. Finally, various strategies for limiting the impact of thermal lensing and thermally induced birefringence on laser performance in simple end-pumped cavity configurations are reviewed.

https://iopscience.iop.org/article/10.1088/0022-3727/34/16/302/pdf

Thermal effects and their mitigation in end-pumped solid-state lasers

Thermal lensing in an end-pumped Nd:LiYF/sub 4/ rod, under lasing and nonlasing conditions, has been investigated Under lasing conditions, a weak thermal lens, with dioptric power varying linearly with pump power, was observed Under nonlasing conditions, where higher inversion densities were involved, hence relevant to Q-switched operation or operation as an amplifier, a much stronger thermal lens was measured, whose power increased nonlinearly with pump power This difference has been attributed to the increased heat deposition due to the subsequent multiphonon decay following various interionic upconversion processes, which increase strongly under nonlasing conditions, and is further exacerbated by the unfavorable temperature dependencies of heat conductivity and the rate of change of the refractive index with temperature A strategy for reducing upconversion and its associated thermal loading, without degrading laser performance, is discussed

/pdf/energy-transfer-upconversion-and-thermal-lensing-in-high-2fzx6w0ifi.pdf

Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals

Longitudinal diode laser pumping causes pronounced inhomogeneous heating of the laser material. The effect of the thermal load on the mechanical and optical properties is significantly influenced by the method used for cooling the pumped surface. Four methods for longitudinally pumped Nd:YAG rods were compared experimentally and with finite-element (FE) analysis. The pumped surface exposed to air or to flowing water, a sapphire plate pressed onto the pumped surface and composite rods, having an undoped pumped end, were investigated. The comparison includes temperature and stress distributions with the emphasis on thermal lensing. Careful validation of the FE code with experiments allowed to numerically comparing the cooling methods under identical conditions.

Cooling schemes for longitudinally diode laser-pumped Nd:YAG rods

The thermally induced beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods were analyzed and the influence of edge- and face-cooling was investigated. The distributions of temperature, stress, and strain in the crystals were calculated by finite element analysis. Based on these data, the space-resolved changes of the refractive index were determined considering thermal dispersion, surface deformation, and strain-induced birefringence. The resulting optical path difference for one round-trip in the end-pumped rods was integrated numerically. For each rod, the induced thermal lens was determined over the extent of the pump spot radius. The calculations of the optical path difference were experimentally confirmed by investigations using a modified Twyman-Green interferometer with a polarized HeNe probe beam at 633 nm under lasing and nonlasing conditions. >

C. Pfistner

Papers

Thermal beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods