Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser

The frequency comb created by a femtosecond mode-looked laser and a microst ructured fiber is used to phase coherently measure the frequencies of both the Hg+ and Ca optical standards with respect to the SI second. We find the transition frequencies to be f(Hg) = 1 064 721 609 899 143(10) Hz and fc, = 455 986 240 494 158(26) Hz. respectively. In addition to the unprecedente d precision demonstrated here, this work is the precursor to all-optical at omic clocks based on the Hg+ and Ca standards. Furthermore, when combined w ith previous measurements, we find no time variations of these atomic frequ encies within the uncertainties of /(partial derivativef(Ca)/partial deriva tivet)/f(Ca)/ less than or equal to 8 X 10(-14) yr(-1) and /(partial deriva tivef(Hg)/partial derivativet)/f(Hg)/ less than or equal to 30 X 10(-14) yr (-1).