Anisotropic self-lasing in large-aperture Ti:sapphire crystals

Summary form only given. The Ti:sapphire lasers of a 100 TW-Petawatt power level need active crystals of 30-100 mm diameter. Because of high amplification gain those crystals are subjected to a high parasitic lasing. We have currently upgraded our Ti:sapphire laser system for 30 TW in a 10-Hz mode and 100 TW in a single shot mode. The last power stage of the laser uses a 35 mm diameter and 15 mm thick Ti:sapphire crystal pumped by frequency doubled pulses of the Nd:glass laser. Up to 30 J of the pump energy can be delivered to the Ti:sapphire crystal. Despite the crystal cladding, a parasitic self-lasing started in the Ti:sapphire crystal when the pump energy exceeded 20 J (pump density 2.8 J/cm/sup 2/) thus limiting the output energy to /spl sim/7 J. The further increase in the pump energy leads to destruction of the originally uniform cladding layer due to absorption of radiation by the layer. We have found that the process of the destruction has two stages. In the first stage, after the first laser shot, only a half of the layer was removed in the direction perpendicular to the optical axis of our crystal. In the second stage, after the next successive laser shot, the rest of the layer was removed also. Development of the parasitic lasing in the Ti:sapphire crystals depends strongly on the aspect ratio of the pumped region. The threshold for parasitic lasing depending on the level of crystal scattering on the side surface of the crystal and on the aspect ratio is presented. Assuming that the scattering on the side surfaces of the Ti:sapphire crystal is close to the normal incidence reflection (7%), the parasitic lasing starts at the aspect ratio values around 0.8-1. In our case the pumped area had an aspect ratio of 2 which is well above the lasing threshold.