Absolute Frequency Measurements of the Hg⁺ and Ca Optical Clock Transitions with a Femtosecond Laser

The frequency comb created by a femtosecond mode-locked laser and a microstructured 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 = 1064721609899143(10)Hz and f_Ca = 455986240494158(26)Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg⁺ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of |(∂f_Ca/∂t)/f_Ca|≤8×10^-14yr^-1 and |(∂f_Hg/∂t)/f_Hg|≤30×10^-14yr^-1.