Optical frequency standards

Abstract

Frequency standards operating in the optical range promise very low frequency uncertainties due to the high spectral resolution and the small sensitivity to external fields. Basic ideas and concepts for the development of optical frequency standards are reviewed with emphasis to laser cooling and ion storing. Utilizing cooled absorbers, potential fractional frequency uncertainties below 10/sup -15/ are envisaged. Phase-coherent frequency measurements of laser radiation at lambda =3.39 mu m and a concept for an extension towards the visible range are discussed. A Ca optical frequency stabilized to the intercombination line ( lambda approximately=657 nm) utilizing Ramsey excitation of a single recoil component is described. The present estimated fractional uncertainty to find the Ca line center is about 2.3*10/sup -12/. It can probably be improved significantly by reducing phase shifts of the Ramsey excitation and by atomic beam cooling. >