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 fHg = 1 064 721 609 899 143\(10\) Hz and fCa = 455 986 240 494 158\(26\) 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 \|\(∂fCa/∂t\)/fCa\|<=8×10- 14 yr-1 and \|\(∂fHg/∂t\)/fHg\|<=30×10- 14 yr-1.