Interference study of a diode-pumped Nd : GGG active disk

We present the results of interference studies of a diode-pumped active disk element made of a gadolinium — gallium — garnet (GGG:Nd) crystal. The disk is cut perpendicular to the crystallographic axis [001], along which the pump beam propagates. With absorbing pump radiation, a thermal lens, which has been investigated by the interference methods using linearly polarised probe radiation, is formed in the disk. At the absorbed pump power up to 12 W, the interference pattern near the disk surface is a system of concentric rings. With increasing absorbed pump power up to 24 W, the outer rings transform into hexagons. It is shown that such an interference pattern in the garnet crystal is caused by thermally induced birefringence.