The first power amplifier receives the pulses after they leave the stretcher. A 10 mm thick, 10 mm diameter Titanium-Sapphire (TiS) crystal is pumped on both sides with a total of 90 mJ of green light from a frequency-doubled Q-switched Nd:YAG laser. The pulse is sent through the crystal four times, and reaches a maximum output energy of 4 mJ after the final pass. After amplification the beam is spatially filtered to smooth its intensity profile and then expanded to 6 mm diameter. At this point a Pockels cell between a pair of crossed polarizers is used to minimise the amount of amplified spontaneous emission (ASE) transmitted to the second amplifier.
Alignment
The alignment of Amplifier 1 is controlled by an automated system in which the beam position is monitored at key points by cameras. The measured spot positions are analyzed in software, and the data is used to control piezoelectric actuators on some of the mirrors in a closed-loop servo configuration. This ensures that the alignment of the amplifier and stretcher remain consistent throughout the day, compensating for changes that occur as a result of temperature variations in the room.
Improving performance
Amplifier 1 was recently rebuilt to improve its performance, in particular to eliminate internal reflections in optics that were degrading the contrast of the compressed pulse. Double reflections in plane-parallel optical components propagate in the same direction as the main pulse, and can give rise to contrast problems. To eliminate these effects, wedged components and Brewster angled windows were used wherever possible. In the new design the spatial filters, that provide image relaying between passes, use Brewster windows for minimum reflectivity, and the TiS crystal has a 10 arc minute wedge between its faces. The orientation of the beam is controlled so that the wedge does not introduce any overall angular or spatial dispersion in the beam.
