High-efficiency deep-UV output from a diode-pumped alexandrite laser

In this paper, we demonstrate for the first time, to the best of our knowledge, the generation of deep ultraviolet (DUV) by third harmonic generation (THG) of a diode-pumped alexandrite laser. A diode-pumped cavity-dumped Q-switched alexandrite laser operating at wavelength 768 nm and 5 kHz pulse rate was converted by sequential second harmonic generation (SHG) to the UV wavelength 384 nm, and then sum-frequency mixing to the DUV wavelength 256 nm. A systematic study was performed of the SHG stage comparing Type-1 LBO and BBO crystals, with different lengths and focal spot sizes, and in walk-off compensated crystal-pairs for spatial quality improvement. Using a walk-off compensated LBO crystal pair, SHG at 384 nm was produced with pulse energy 50 µJ (32% conversion efficiency) and THG at DUV wavelength 256 nm with pulse energy 30 µJ (19% conversion efficiency with respect to fundamental). This study shows excellent promise for efficient generation of tunable UV and DUV sources using the broad spectral near-visible lasing bandwidth of diode-pumped alexandrite lasers.