Benefiting from their unique structures, Innoslab lasers have achieved high power and high beam quality output. However, there is still a lot of harmful heat in the laser crystal under the high-power pumping. To obtain an amplification output with high beam quality, the heat in the laser crystal during the laser amplification process must be effectively removed. An indium layer is added between the laser crystal and the heat sink before amplification. During this process, defects such as voids are inevitably introduced, which will inhibit the heat dissipation. Heat is accumulated in the crystal because of defects in the indium layer, resulting in wavefront distortion that leads to poor beam quality and even the crystal fracture due to inhomogeneous thermal stress distribution. Therefore, it is necessary to analyze the effects of defects characteristics on the temperature distribution inside the crystal. In this paper, the influences of the sizes and positions of the indium layer defects and the distance between two adjacent defects on the crystal temperature distribution are quantitatively investigated.

In this paper, the effects of the sizes and positions of the indium layer defects and distance between two adjacent defects on the crystal temperature distribution are studied by the finite element method using the COMSOL software. Firstly, the heat source distribution model is obtained based on the distribution of pump light and the absorption characteristics of the crystal, and the temperature distribution inside the crystal is got through the steady-state heat conduction equation. After that, the temperature distribution of the crystal without defects in the indium layer is obtained. Then, circular defects with different sizes, positions and spacings are introduced into the ideal indium layer to obtain the temperature distribution of the laser crystal with defects in the indium layer. The influences of indium layer defects on crystal temperature distribution are obtained by comparing the temperature distribution of the crystal without and with defects in the indium layer.

The temperature of the crystal in the pumping direction is low in the middle and high at the sides of the crystal without defects in the indium layer, and the highest temperature point of the crystal is near the incident surface of the pump light on the crystal. With a defect inside the indium layer, the temperature rises at the position where the defect is located. As the defect size is increased, the temperature increment value increases, and the highest temperature point shifts from the incident surface of the pump light to the location near the defect (Fig. 5). The effects of the defect on the temperature distribution of the crystal with the defect above the high-temperature region of the crystal are more significant than those of the defect above the low-temperature region (Fig. 6). The influences of two adjacent defects on the crystal temperature distribution are coupled when the distance between the two defects is small, resulting in more significant effects of the defects on the crystal (Figs. 7 and 8).

In this paper, the temperature distribution of crystal with and without defects in the indium layer is simulated, and the distribution characteristics are qualitatively analyzed using the heat conduction equation under steady-state conditions by establishing the heat source distribution model of laser crystal and introducing defects into the indium layer. The highest temperature of the crystal surface obtained by simulation without defects in the indium layer is relatively consistent with the experimental results. At the same time, the influences of the defect size on the temperature distribution in the X, Y and Z directions of the crystal, and the influences of the defect positions and the distance between two adjacent defects on the temperature distribution in the X direction of the crystal are quantitatively analyzed. It is found that the larger the defect size is, the larger the temperature increment value and temperature rise area caused by the defect are, and the highest temperature point in the X direction of the crystal shifts from the crystal end face to the location near the center of the defect when the defect size is larger than 0.65 mm. The influences of the defect in the high-temperature region on the crystal temperature distribution are more significant than those of the defect in the low-temperature region. The effects of two defects that are far apart on the crystal temperature distribution are independent of each other. However, when the defects are close to each other, their effects on the heat dissipation of the crystal are coupled, resulting in more significant effects of the defects on the crystal temperature distribution. The work in this paper is useful in deciding whether or not to consider the influences of indium layer defects on the experiment during the laser design and laser crystal installation, which is of guiding significance for laser design and experiments.