New method approach to estimate the thermal volume flow deposited within the crystal over laser pumping (Conference Presentation)

In recent past years, solid state laser have been attracted attentions in industry and scientific researches to achieve both high average power and energies with good beam quality. As an example, high energy laser with average powers over the kW have been demonstrated [1]. In these systems, predicting and managing heat generated during optical pumping is critical, as it can result in unwanted thermo-optical and thermo-mechanical effects such as thermal lensing or thermal stress fracture. The determination of temperature distribution inside the laser gain medium is essential for optimizing the laser operation. The first step in thermal analysis is to estimate the thermal volume flow deposited within the gain medium over optical pumping, but this very first point is extremely difficult to accurately measure. In this work, we propose a novel method, based on a both an experimental thermal imaging set-up and a new theoretical model, that we use to accurately measure the deposited thermal flux in a laser crystal during pumping. We use a high frequency thermal camera and a pulsed laser diode ( Pump LD 969 nm or 940 nm) to submit a photo-thermal excitation to the front surface of a non-contacted Yb:YAG laser crystal. Based on singular value decomposition and inverse methods, thermal diffusivity and the heat losses were estimated using the thermal evolution of the crystal over the relaxation step. These results and the thermal evolution of the crystal over the solicitation allowed estimating the thermal volume flow deposited within the crystal over optical pumping.