Thermal Lensing Analysis of Alexandrite Laser Rods by Moire Deflectometry

Thermal lensing in solid state laser media is the key to the design and performance of solid state lasers.[1] Its transient character is a complicating feature. When a lasing medium is uniformly excited by optical pumping, the heat generated is dissipated by conduction to the surface boundary layer. The thermal gradient ∇T produces a spatial index gradient ∇n. In a rod geometry, the radial index gradient, dn/dr, gives rise to a thermal lens. The focal length of the thermal lens is determined by parameters, such as the rod absorbance, diameter, length, and by boundary conditions which are determined by the pumping and cooling conditions. The behavior of dn/dr [and thus of n(r,t)] can vary substantially during the time of a pump pulse. The transient thermal response for a pumped ruby rod has been calculated[2] for a single flashlamp pump pulse. Its detailed, theoretical analysis is limited by knowledge of the exact experimental conditions. Experimental[1] and theoretical results for ruby and Nd:YAG laser rods show both divergent and convergent lensing due to the changing distribution of heat with time.