Temporal Characterization and Intensity Contrast Improvement of Few-cycle Laser Pulses

Sammanfattning: Due to the unique combination of properties of high intensity and few optical cycle pulse duration, the ultrashort laser pulses have widespread applications in pump-probe spectroscopy and laser-plasma interaction. Correspondingly, it is critical to precisely measure the electric field in the temporal or spectral domain. Since there are no shorter pulses available to sample them in time, it is not easy to characterize few-cycle pulses. In this thesis, the temporal characterization methods including second harmonic generation (SHG) interferometric autocorrelation, single-shot SHG frequency-resolved optical gating (FROG) and chirp scan are used to measure the laser pulses at different positions in our laser system, the Light Wave Synthesizer-20. The SHG interferometric autocorrelation measured a 25.3 fs pulse duration after the kHz frontend laser with a 24.5 fs Fourier limit (FL). The FROG is tested by measuring seed pulses with a 6.5 fs FL providing 6.7 fs pulse duration with mostly flat phase. Then, this FROG apparatus is used to characterize the amplified pulses with a 4.5 fs FL after the whole laser system. The measured pulse duration is 4.5 fs reaching the FL. Similarly, the chirp scan is also used to measure the amplified pluses with a slightly different spectral phase, which retrieved a longer pulse duration of 5.2 fs. The second part of the thesis is focused on contrast improvement. The temporal intensity contrast is reduced in the amplification process leading to a deteriorated laser-plasma interaction. Contrast improvement based on the nonlinear elliptical polarization rotation (NER) technique in a hollow-core fiber (HCF) is implemented and its optimization is performed by using high extinction ratio polarizer, utilizing Ar gas and testing smaller polarization ellipticity (NER angle). The optimal condition is found to be 7° NER angle and 650 mbar Ar in the HCF. Under this optimal condition, sub-4 fs pulses with a smooth spectrum are generated with a power above 30 mW. The NER efficiency is higher than 50%. After amplification to 75 mJ energy with sub-5 fs duration the measured contrast improvement is 3 order of magnitude. In conclusion, the intense few-cycle pulses have been fully characterized by FROG and chirp-scan techniques. Furthermore, the NER method is promising to get cleaned pulses with higher than 3 order of magnitude contrast enhancement.