Optical frequency measurements using fs-comb generators

We have used a Ti:Sapphire-based fs-comb generator to measure the frequencies of several optical frequency standards available at PTB. The frequency values for HeNe- and frequency-doubled Nd:Yag lasers stabilized to iodine hyperfine transitions as well as the frequencies of the ¹S₀-³P₁ transition of ⁴⁰Ca at 657.5 nm (456 THz) and the ²S₁2/-²D₃2/ transition of ¹⁷¹Yb⁺ at 435.5 nm (688 THz) are presented. Comparing our results with previously published data for the frequency variation of transitions of the Hg⁺ ion and H atom, we have derived a new upper limit of the variation of the fine structure constant α.