Yb fiber amplifier at 972.5 nm with frequency quadrupling to 243.1 nm
  We demonstrate a continuous-wave ytterbium-doped fiber amplifier which produces 6.3 W at a wavelength of 972.5 nm. We frequency quadruple this source in two resonant doubling stages to produce 530 mW at 243.1 nm. Radiation at this wavelength is required to excite the 1S-2S transition in atomic hydrogen and could therefore find application in experimental studies of hydrogen and anti-hydrogen. 
Z. Burkley, C. Rasor, S. F. Cooper, A. D. Brandt, D. C. Yost
physics.atom-ph