Abstract

The Ti:sapphire lasers of a 100 TW—Petawatt power level need active crystals of 30-100 mm diameter. Because of a high amplification gain those crystals are subjected to a high parasitic lasing.^1–3 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.³ The last power stage of the laser uses a 35 mm diameter and 15 mm thick Ti:Sa 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:Sa crystal. Despite of the crystal cladding a parasitic self-lasing started in the Ti:Sa crystal when the pump energy exceeded 20J pump density 2.8 J/cm²) thus limiting the output energy to ~7J. The further increase in the pump energy leads to destruction of the originally uniform cladding layer due to absorption of the 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 (see fig. 1). In the second stage, after the next successive laser shot, the rest part of the layer was removed also. Our explanation of such behavior is based on anisotropic features of Ti:Sapphire crystal. Big difference in the values of stimulated emission cross-section for σ- and π-polarizations leads to big difference in the appropriate small signal gain values. On the other hand, one can see from basics of light and fig. 1, that π-polarized lasing can take place only for the direction, perpendicular to the optical axis. Until the cladding layer exists, internal parasitic resonator inside of the crystal has so high losses, that only π-polarization can reach strong enough energy level. When the cladding layer is partially destroyed, light scattering from unpolished crystal surface leads to filling of the crystal with strong amplified radiation with the full spectrum of directions. That causes destruction of the rest part of the layer.

© 2002 Optical Society of America