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Ultrafast ultraviolet UV lasers have significant applications in biological, industrial, and other fundamental research. For example, ultrafast laser with wavelength at 257 nm is the photon-injector driving source for next generation high repetition rate free-electron laser facilities. In this manuscript, we investigate the fourth harmonic generation of post-compressed IR pulses and suggest a high-performance sub-100 fs UV pulse generation method. Self-phase modulation plays a major role in spectral broadening in the post-compression, allowing the FHG to have a broader spectrum and achieve a shorter pulse duration, which will shrink the footprint of pulse stretching afterwards by a factor of 5. In this study, we employ a commercial laser as the fundamental source, which generates 50 μJ pulse energy, and 350 fs pulse duration at wavelength of 1030 nm. First, the pulse duration is compressed down to 66 fs after thin plate compression. Then we theoretically study the FHG after the nonlinear compression. The simulation shows, A UV pulse of 3.43 μJ, 118 fs was finally produced using two cascaded BBO crystals with thicknesses of 3 mm and 0.5 mm. As an alternative, a UV pulse lasting 1.37 μJ, 98 fs was produced using two BBO crystals with thicknesses of 3 mm and 0.2 mm. Additionally, we discovered that this approach does not result in a decrease in the FHG conversion efficiency compared with the un-compressed pulse.
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