CA OPTICAL FREQUENCY STANDARD

Two versions of a Ca optical frequency standard are described. The fir st one is transportable, utilizing an effusive beam of Ca atoms, which are excited by a high-resolution diode laser. The relative uncertaint y of its frequency is below 2 x 10(-12). It is limited by residual Dop pler shifts of the thermal atoms. These shifts are strongly reduced in the second version, which utilizes laser-cooled and trapped Ca atoms. Two standards based on trapped Ca atoms gave the same frequency withi n approximately one part in 10(13). Phase-coherent measurements of the optical frequency result in a mean value of nu(Ca) = 455986240494.15 kHz with a relative standard uncertainty of 2.5 x 10(-13).