Abstract

The amplification, at a repetition rate rather high (10 Hz or more), of femtosecond pulses in the Titanium doped Sapphire at an energy level of 3.5 joules, in order to obtain focused intensities in the range of 10²⁰ W/cm², rely on the use of 80 Watts of average pump power. A part of the absorbed power is transformed in heat and produces some thermo-mecanical modifications on the gain medium. This leads to severe changes in the spatial characteristics of the amplified beam. A solution on which we can rely on is the use of a negative lens in the amplifier. This solution enables to overcome the thermal lens but not the others thermal aberrations such as thermal astigmatism. Instead of trying to compensate for a part of the thermal effects, we have chosen to reduce them by cooling the Ti:Sa crystal to cryogenic temperatures (typically 140°K) [1] where the crystal exhibits better thermal properties [2,3].

© 2000 IEEE