  We demonstrate a 1.4 W continuous wavelength (CW) laser at 243.1 nm. The
radiation is generated through frequency quadrupling the output of a
ytterbium-doped fiber amplifier system which produces $>$ 10 W of CW power at
972.5 nm. We demonstrate absolute frequency control by locking the laser to an
optical frequency comb and exciting the 1S-2S transition in atomic hydrogen.
This frequency-stabilized, high-power deep-UV laser should be of significant
interest for precision spectroscopy of simple and exotic atoms, two-photon
laser cooling of hydrogen, and Raman spectroscopy.
