Continuous Wave Thermal Loading in Saturable Absorbers: Theory and Experiment

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Abstract
Theoretical and experimental results of a study that investigates cw thermal loading in solid-state saturable absorbers with low heat conductivities are presented. In addition to the temperature dependence of the refractive index, the proposed model considers the temperature dependence of the fluorescence lifetime to account for the local variations in the saturation intensity resulting from thermal gradients. In the calculations an iterative scheme is employed to calculate first the temperature distribution produced by the pump beam subject to saturable absorption with a constant saturation intensity and then the resulting modifications in the propagation parameters that are due to the presence of the calculated temperature distribution. Excellent agreement is obtained between the numerically calculated results and experimentally measured cw transmission data obtained with use of a Cr:YAG saturable absorber. Because the absorption cross section of the medium is used as one of the fitting parameters to yield the best fit between theory and experiment, the model further offers an accurate method whereby the cw power transmission data can be used to determine the absorption cross section of a saturable absorber subject to thermal loading.