The second power amplifier is arranged in a “bow-tie” configuration, in which the beam path takes the pulses through the crystal four times. The crystal itself is a 10 mm thick piece of Titanium-Sapphire (TiS), pumped on both sides by 300 mJ pulses of green light from a frequency-doubled Q-switched Nd:YAG laser. The pump beams are image-relayed from the source to give a 6 mm diameter gain region with a uniform intensity profile. The infra-red pulses are amplified to an energy of 120 mJ in this amplifier. After amplification, the beam is expanded to 18 mm and apodized to give a quasi-flat-topped beam profile.
Providing a plasma diagnostic beam line
After expansion, part of the beam (around 10 %) is split off from the expanded beam to provide a probe beam for target area 2, where it is used to generate ultrashort optical pulses for probing the experimental plasmas. A small fraction of the split-off beam is also used to diagnose the main beam profile and focal spot at the output of the amplifier. The remainder of the beam continues to the third power amplifier.
Improving performance
Amplifier 2 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. The beam splitter that provides the probe beam for the target area has a small wedge angle, and immediately following it there is a compensating wedge with an equal but oppositely-orientated angle, to eliminate angular dispersion that would prevent proper compression of the pulse.
