High Repetition Rate Alexandrite Laser Divergence Measurements

Beam divergence measurements were performed on a high repetition rate alexandrite laser currently being developed for a Los Alamos National Laboratory photochemistry research program, and were found to be spherically correctable and approximately constant at 8 times diffraction limited over a large input power range. A 0.5 x 10 cm alexandrite rod was pumped in a double ellipse head at a constant 42 joules/pulse input energy at repetition rates of 75 to 200 Hz. Several resonators were employed over the input power range to compensate for the thermal lensing, which varied from 0.63 to 0.12 m. The divergence measurements were performed by splitting a fraction of the output beam, passing it through a long focal length lens, and measuring the transmission percentage through calibrated apertures at the focal plane. This measurement was performed for seven resonators and cross-checked by imaging the far field pattern through a TV camera system and observing the spot sizes. With a similar experimental setup, a Glan prism was placed extra-cavity to examine the magnitude of depolarization losses due to stress induced birefringence under pumped conditions. No measurable effect was found up to 1 kW input power.