## 3 Results and Discussion
### Temporal Pulse Structure  
In Figure 4 an overview of the temporal pulse structure of the SHG-pulses is shown in a log-log representation of focussed intensity vs. time. The starting point was a measurement carried out for the 1030 nm-pulses of POLARIS. We want to point out that the measurement method leading to Figure 4 is described in detail in [25] and the experimental data for the used 1030 nm laser pulses are published in section 6 of [10]. To estimate the on-target temporal intensity contrast we have to take two influences into account. Firstly the temporal contrast of the SHG-pulses and secondly the temporal contrast of the fundamental beam. The black curve in Figure 4 displays the intensity of the SHG-pulses on a ps-timescale and was calculated by taking the square of a cross-correlation measurement (Sequoia, Amplitude) of the fundamental pulses. Due to the SHG, which is a \(2^{\mathrm{nd}}\)-order nonlinear process, the intensity of the newly generated signal is the square of the input and this conversion is acceptable. An identical conversion was made with a self-referenced spectral interferometer (SRSI, Wizzler, Fastlite) measurement and is shown in the blue curve. Note that the measurement with the Sequoia is taken in the near-field of the compressed pulse and since we are using a tiled-grating compressor some spectral components of the  
Figure 4: Measured and calculated temporal characterization of the on target intensity taking SHG and fundamental pulses into account. Negative times are defined as the times before the laser pulse. Black: converted THG-correlation for 515 nm. Blue: converted self-referenced spectral interferometry (Wizzler, Fastlite) measurement for 515 nm. Red: converted THG-correlation for 1030 nm. Ocher: Converted photodiode measurement for the 1030 nm frontend amplified spontaneous emission (ASE). Dark blue: Converted photodiode measurement for the 1030 nm multipass ASE. The detection limit of each measurement is marked as a dashed line. pulses are missing depending on the lateral position in the laser pulse. This may lead to a smaller relative intensity contrast for the laser pulse (and its leading edge) as compared to the contrast in the focal spot where the pulses are applied in experiments [10]. Summarizing this argument means that the temporal region \(\pm 1.5\) ps around the main pulse is likely better represented by the SRSI-measurement. For the sake of completeness we note that the SHG signal of the POLARIS frontend and multipass ASE is far below the intensity range in Figure 4.  
Using a collinear type-I phase matching SHG the fundamental beam co-propagates with the SHG-pulses and could only be attenuated stepwise. For the attenuation we used two wavelength separators (_cf._ Section 2.1) with a total extinction ratio of \(1\times 10^{-6}\). Taking into account the pulse duration, the focal spot size and the energy loss of the fundamental beam during the SHG we calculated a peak intensity reduction of \(1\times 10^{-5}\) for the attenuated fundamental beam with respect to the focussed SHG-pulses. The attenuated fundamental beam is displayed in Figure 4 with three different curves. At first the multipass ASE (dark blue) starting on a ms-timescale before the main pulse with a relative intensity of \(\approx 10^{-22}\), secondly the frontend ASE (ocher) starting some ns before the main pulse with an intensity level of some \(10^{-18}\) and finally the main pulse (red) measured with a third-order correlator (Sequoia, Amplitude). Note that the detection limit in the red curve is reached \(\approx\)100 ps before the main pulse and all three curves are normalized to the maximum peak intensity of the SHG-pulses. It is obvious in Figure 4 that on ms- and ns-timescales the ASE of the fundamental pulse determines the temporal intensity contrast to values better \(10^{-17}\) until the leading edge of the fundamental pulse and the SHG-pulse \(\approx\)100 ps before the main pulse plays the dominant role.