<title>Optical frequency standards and measurements of optical frequencies</title>

Abstract

The development of precision optical frequency standards is reviewed with emphasis to standards based on laser cooled and trapped atomic ensembles. As a typical example, an optical frequency standard based on laser-cooled and stored Ca atoms is presented. The use of optical frequency standards requires the knowledge of their frequencies. Methods and proposals for the synthesis of optical frequencies like harmonic mixing, mixing of subharmonics, cascaded frequency interval division, and wide band frequency modulation are discussed. A phase-coherent frequency measurement of the Ca optical frequency standard is demonstrated representing the first phase-coherent frequency measurement of visible radiation related directly to the primary standard to time and frequency.