Yb-fiber-laser-based, 18  W average power, picosecond ultraviolet source at 266  nm

We report a compact, stable, high-power, picosecond ultraviolet (UV) source at 266 nm based on simple single-pass two-step fourth-harmonic generation (FHG) of a mode-locked Yb-fiber laser at 79.5 MHz in LiB3O5 (LBO) and β-BaB2O4. Using a 30-mm-long LBO crystal for single-pass second-harmonic generation, we achieve up to 9.1 W of average green power at 532 nm for 16.8 W of Yb-fiber power at a conversion efficiency of 54% in 16.2 ps pulses with a TEM00 spatial profile and passive power stability better than 0.5% rms over 16 h. The generated green radiation is then used for single-pass FHG into the UV, providing as much as 1.8 W of average power at 266 nm under the optimum focusing condition in the presence of spatial walk-off, at an overall FHG conversion efficiency of ∼11%. The generated UV output exhibits passive power stability better than 4.6% rms over 1.5 h and beam pointing stability better than 84 μrad over 1 h. The UV output beam has a circularity of >80% in high beam quality with the TEM00 mode profile. To the best of our knowledge, this is the first report of picosecond UV generation at 266 nm at megahertz repetition rates.