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Analysis of thermo-optic effects in Nd:YAG ceramics disk under high heat load

The aims of paper were theoretical analysis of thermo-optic effects occurring inside laser elements under high heat load and its experimental verification for two particular cases: dichroic window and Nd:YAG ceramics disk. Transient thermal effects in dichroic mirrors and ceramic gain media were modeled applying COMSOL Multiphysics software and verified experimentally. Moreover, thermally induced distortions, thermally induced stresses and birefringence were calculated for gain elements of rod and disk shapes applying analytical, stationary model based on linear thermoelasticity theory. The 100-mm diameter dichroic mirrors made of BK7 and fused silica and gain disks made of Nd:YAG ceramics of 15-mm diameter and 3-mm thickness were prepared for experimental verification of the theoretical models. The special laboratory set-up enabling simultaneous registration of thermally induced birefringence and wavefront distortions was worked out. We have investigated the thermo-optical effects for different heat densities in range of 0.1 kW/cm2 up to 50 kW/cm2 changing the pump power , beam diameter or duty cycle. The experiments were carried out in lasing and non lasing conditions. The new method of measurement of heat conversion efficiency and absorption in mirrors based on threshold shearing interferometry was proposed and verified for dichroic mirror and ceramic Nd:YAG disk.