# A Versatile 100 Hz Laser System with Few-Cycle and TeraWatt Pulses for Applications
## 5 The TW Laser
### Amplifier Stages  
The two Ti:Sa amplifiers share the 10 ns, 532 nm pump pulses with an energy of 200 mJ from a diode-pumped, frequency doubled Nd:YAG laser (Plasma 450-100 by Litron, Rugby, Warwickshire, UK). For each amplifier stage, the timing of the pump pulse from the side of the first pass seed pulse is optimized for its highest amplification, while the other pump timing from the other side is optimized for the highest overall gain. As a result, the delay between the two pump pulses is about 2 ns. This optimization method, resembling the extraction during pump scheme [14], allows for the use of somewhat smaller beam sizes in the Ti:Sa crystal, and hence the peak fluence on the Ti:Sa crystal is lowered, but the overall gain is maintained. The five-pass (Figure 6a) booster amplifier (BA) increases the incoming 0.14 mJ energy to 8 mJ, with an energy stability of 1.7% (RMS). The four-pass (Figure 6b) power amplifier (PA) provides a gain of almost 25, resulting in a pulse energy of 75 mJ. Due to the saturated amplification of the initial blue shaped spectrum, the outcoming spectral shape is red-shifted (Figure 7).  
Figure 6: The schematics of the five-pass booster amplifier stage, including elements of continuous, spectral, energy, and beam size monitoring (**a**); and the four-pass power amplifier (**b**).  
We also simulated the operation of the amplifier stages based on the Frantz-Nodvik model [4; 21] adapted for broadband stretched pulses [22] and we took into account the spectral dependence of the emission cross-section of Ti:Sa [23]. The measured pump and seed pulse properties (energy, beam size, spectra) were used as input parameters. In the amplifier model, the spectral evolution was further affected by the spectral phase modulation of the seed pulses, and the spectral reflectance of the amplifier optics. The mode size of the pump and seed pulses was set according to the experimental conditions. By tuning the spatial overlap of the seed and the pump in the Ti:Sa crystal, the effective pump fluence was varied according to the experimentally observed output of each stage. We accurately reproduced both the amplified spectra and the pulse energies at the output of both the BA and PA stages with 8 mJ and 72.5 mJ output energies, respectively (Figure 8, PA measured and PA simulated). Due to the positive temporal stretch, the initially blue-shaped spectrum shifted towards the longer wavelengths, whereas the red side of the spectrum increased, in line with the expectation under saturated amplification.  
The amplified output pulses were further broadened introducing a spectral dip into the spectrum by the AOPDF in the FE, resulting in a spectral shape close to super-Gaussian.  
Figure 8: Spectral evolution in the amplifiers: measured seed (gray area), measured output of the power amplifier (orange), and simulated power amplifier output (dark gray dashed) with spectral shaping in the frontend.  
Figure 7: Spectral evolution of the pulses in the laser system. The spectrum of the negatively stretched FE pulses is blue shifted, while the positively chirped pulse amplification in the saturated BA and PA compensates for it, resulting in a bandwidth of 44 nm.  
With such a spectrally pre-shaped input pulse to the BA, the simulated amplified spectrum agrees well with the measured one. This shows not only the power of the simulation model, but also provides a possibility to find the optimum pre-shaping for the output spectrum of any application.