Thermally Induced Focusing in a Nd:YAG Laser Rod at Low Input Powers

Thermally induced focusing in a neodymium-doped yttrium aluminum garnet rod was measured for pulsed optical pumping conditions with low average pump powers (400 W or less). A transient defocusing and a quasi-steady-state equilibrium with positive focusing are identified. The equilibrium induced focusing is shown to depend only on the average pump power for pulse rates above about 5 Hz. The quantitative dependence of the equilibrium induced focusing power of the rod on average pump power was measured, and effects of this induced focusing on the parameters of a laser resonant cavity containing the rod were investigated. A technique for accurately measuring focal powers of the order of 0.02 diopter in systems with f-numbers as high as 8000 is described.