Absolute frequency measurement of the 435.5-nm Yb-171(+)-clock transition with a Kerr-lens mode-locked femtosecond laser

We have measured the frequency of the 6s S-2(1/2) - 5d(2)D(3/2) electric-qu adrupole transition of Yb-171(+) with a relative uncertainty of 1 x 10(-14) , nu (Yb) = 688 358 979 309 312 Hz +/- 6 Hz. We used a femtosecond frequenc y comb generator to phase-coherently link the optical frequency derived fro m a single trapped ion to a cesium-fountain-controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencie s ever reported, and it represents a contribution to the development of opt ical clocks based on a Yb-171(+)-ion standard. (C) 2001 Optical Society of America.