Efficient, high energy regenerative amplification

Summary form only given. We report on a diode-pumped laser regenerative amplifier design capable of delivering energies in the 100-500 mJ range with good extraction efficiency. We recently demonstrated a 100 mJ regenerative amplifier and we present our latest results in the 300 mJ-500 mJ range. In order to achieve such a high level of energy, several issues have to be faced. First, the pumping energy has to be high. For this reason, we side pump the amplification medium with stacks of 8 diode bars delivering 150 mJ in 400 /spl mu/s, for a peak current of 80 A. We use Nd:phosphate glass as an amplification medium because of its spectral properties but its poor thermal conductivity restricts the laser repetition rate to 1 Hz. At such a low rate, no external cooling is applied to the laser head. Then, the amplification medium size is chosen to achieve a stored fluence of the order of its saturation fluence. This is done to ensure good regenerative amplification. This results in the use of square amplification rods with sizes between 3.5 and 7 mm, depending on the available pumping energies Second, a phase plate is used to shape the laser mode in order to fit at best the amplification rod size and therefore increasing the extraction efficiency. Experimental measurements have shown above 10% energy conversion efficiencies, which is comparable to the results achieved by other pumping strategies.