A critical parameter in experiments employing ultra-intense lasers is the intensity contrast. Prepulses with considerable intensities may result in failure of experiments. In an experiment with expected peak intensity of 1021 W/cm2 in focus, a prepulse with 5-6 orders of magnitudes less intense than the main pulse is still high enough (1015-1016 W/cm2) to induce parasitic effects to the experiments. In PW class systems, a contrast better than 10-8 is required at delays of tens to hundreds of ps.
Measurements of such large magnitudes for short delays are possible by autocorrelation. The device available at CETAL for contrast measurements is TUNDRA (UltraFast Innovations GmbH), a high dynamic range equipment capable theoretically to measure an intensity contrast down to 10-10. TUNDRA is based on the 3th order autocorrelation. In such configuration, a SGH signal from a frequency doubling non-linear crystal and the fundamental beam are temporally synchronized and spatially superposed in a THG crystal. The two incident beams are non-collinear on the nonlinear crystal. The resulting THG beam is formed as the vectorial sum of the kω and k2ω wave vectors.
Measurements of such large magnitudes for short delays are possible by autocorrelation. The device available at CETAL for contrast measurements is TUNDRA (UltraFast Innovations GmbH), a high dynamic range equipment capable theoretically to measure an intensity contrast down to 10-10. TUNDRA is based on the 3th order autocorrelation. In such configuration, a SGH signal from a frequency doubling non-linear crystal and the fundamental beam are temporally synchronized and spatially superposed in a THG crystal. The two incident beams are non-collinear on the nonlinear crystal. The resulting THG beam is formed as the vectorial sum of the kω and k2ω wave vectors.
By scanning the delay between fundamental beam and SHG, the autocorrelation signal is recorded as the intensity of the THG.
When the peaks are temporally synchronized, a high intensity SHG with low intensity of fundamental results in a high intensity THG.
Also low intensity SHG and high intensity of fundamental gives a low intensity THG.
Then, if the peak at negative delay was higher amplitude compared with its replica from positive delay, that peak is a real pre-pulse.
But, if it is at the same amplitude or less than the positive delay replica, it is a post-pulse.
Ultra high-contrast third-order autocorrelator Tundra (UltraFast Innovations GmbH)
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