## 4 Results and Discussion
### Compression of Self-Phase Modulated UV Pulses  
The availability of sub-25 fs UV pulses generated by parametric amplification and conversion is in general limited in optic laboratories. More often the third harmonic of the Ti:sapphire system is used directly. To show that SPM can be used in such a configuration we employ the third harmonic of our CPA laser as the initial pulse. Pulses centered at 257 nm with an energy of about 1.5 \(\upmu\)J are spectrally broadened in a 1 mm thick CaF\({}_{2}\) crystal (Figure 5). The pulses are subsequently compressed by a UV-PC and then measured with the UV autocorrelator. When the full spatial beam profile is used, we obtain a reduction of the initial Fourier limit of 80 fs down to 49 fs (Figure 5a). The measured autocorrelation trace (Figure 5b) shows that the Gaussian shape of the input pulse is conserved. The measured pulse length FWHM changes from 120 fs to 73 fs. When only the central part of the broadened pulses is selected by an aperture (pulse energy reduced by 30%), we obtain a much broader spectrum, as the spatial intensity is higher in the central part. The initial Fourier limit of 80 fs is shortened by nearly a factor of 3 down to 29 fs (Figure 5c). The measured pulse length is 50 fs (Figure 5d). The autocorrelation trace of the broadened pulses shows deviations from a Gaussian shape due to higher order dispersion which could be eliminated by a more sophisticated compression scheme.  
To show that the spectrally broadened pulses are compressible near to the Fourier limit, we employed an UV-PC in combination with an AOPDF to compensate for all orders of dispersion. We compressed SPM broadened pulses centered at 273 nm, generated by frequency doubling the output of the NOPA. Due to the low input energy of 0.7 \(\upmu\)J, we used a 2 mm thick CaF\({}_{2}\) crystal for broadening. The recollimation lens L2 was moved in order to compensate for the SPM induced change in beam divergence while the specific collimation requirements of the AOPDF are met (see ref. [1] for details). According to the literature [17], for a Gaussian beam profile SPM can lead to different frequency chirps across the transverse beam profile related to the spatial intensity profile. Therefore an aperture was used to block the outer parts of the beam. This reduced the pulse energy by about 20%. In order to compensate the chirp of the pulse, the spectral phase was measured by a ZAP-SPIDER and the obtained phase function was then recursively used in the AOPDF to completely flatten the phase. The results are shown in Figure 6a and b together with the temporal profile. We are able to obtain a flat spectral phase and the pulse length is reduced to 19.8 fs, which is close to the Fourier limit of 19 fs.  
Figure 5: Pulse compression of the 120 fs third harmonic of the CPA by SPM broadening in a 1 mm CaF\({}_{2}\) crystal and chirp compensation by a prism compressor. Dotted lines: input pulses; solid lines: spectrally broadened pulses. Spectra (**a**, **c**) and autocorrelation measurements (**b**, **d**) of the complete beam profile (blue lines) and the central part of the beam profile (red lines).  
For the pulses with a central wavelength at 313 nm generated by sum frequency mixing the thickness of CaF\({}_{2}\) disk was changed to 1 mm as more energy (2.1 uJ) was available. The broadened pulses can again be compressed well below the input Fourier limit to 14.9 fs. This corresponds to a reduction of the pulse length by a factor of about 1.5 of the initial 22 fs long pulses. The energy after the PC was typically about 2 uJ and that of the compressed pulses after the shaper was about 200 nJ, which corresponds to the 10% overall efficiency of the shaper device. This is an order of magnitude higher compared to the output energies obtained previously with the AOPDF [1].