Abstract
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 /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition of /sup 40/Ca at 657.5 nm (456 THz) and the /sup 2/S/sub 1/2/-/sup 2/D/sub 3/2/ transition of /sup 171/Yb/sup +/ at 435.5 nm (688 THz) are presented. Comparing our results with previously published data for the frequency variation of transitions of the Hg/sup +/ ion and H atom, we have derived a new upper limit of the variation of the fine structure constant /spl alpha/.
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