Optical frequency standards based on the 199Hg+ ion

We report on work directed toward the systematic evaluation of an optical frequency standard based on the 2 1 25 2 transition of a single, laser-cooled, trapped Hg + ion, whose resonance frequency is 1.06 10Hz. For the purpose of the evaluation, a second 199Hg standard has been constructed. In the cooling-laser system built for the second standard, an injection-locking scheme has been applied to a CW Ti-sapphire laser. We also report optical frequency measurements of the clock transition performed over the past 21 months with the first standard. During this term, the variation of the clock transition frequency is found to be less than 1 10 .