A transportable optical calcium frequency standard

We report on the development and characterization of a transportable freque ncy standard to disseminate one of the most precisely known optical frequen cies, the frequency of the Ca intercombination line at 657 nm. The key comp onents of the transportable standard are a pre-stabilized diode laser syste m with a laser linewidth of below 2 kHz and a miniaturized thermal calcium beam apparatus. A separated-field (optical Ramsey) excitation scheme was us ed to resolve spectroscopic structures of a width of below 20 kHz, which we re used to stabilize the laser frequency to the center of the intercombinat ion line. Procedures were developed and applied to reduce effectively the i nfluence of the Doppler effect on the frequency accuracy in the first and s econd order. The characterization of the stability and reproducibility of t he standard was investigated in comparison with a stationary standard based an laser-cooled Ca atoms resulting in a relative frequency uncertainty of 1.3 x 10(-12).