The change of the optical path length in laser rods, under the process of flash optical pumping, depends on the temperature increase of the laser material due to an absorption of pump and laser radiation, on stress-optical effects, and on the varying population of the levels of the active ions. A quantitative comparison of these three effects has shown that the change of the optical path-length is predominantly determined by temperature variations. Thermal effects have been experimentally investigated by an interferometric technique. The dependence of the optical path-length variations within the pumping period on pump energy, flash time, pumping arrangement, geometrical configuration, and doping concentration of the laser rod is analyzed for ruby and Nd+++ doped glass laser rods.
HERBERT WELLING and CHARLES J. BICKART, "Spatial and Temporal Variation of the Optical Path Length in Flash-Pumped Laser Rods," J. Opt. Soc. Am. 56, 611-618 (1966)