title: Phase coherent vacuum-ultraviolet to radio frequency comparison with a mode-locked laser
abstract: We demonstrate a versatile new technique that provides a phase coherent link between optical frequencies and the radio frequency domain. The regularly spaced comb of modes of a mode-locked femtosecond laser is used as a precise ruler to measure a large frequency gap between two different multiples (harmonics or subharmonics) of a laser frequency. In this way, we have determined a new value of the hydrogen 1S-2S two-photon resonance, f(1S-2S) = 2 466 061 413 187.29(37) kHz, representing now the most accurate measurement of an optical frequency.
authors.affiliation: Max-Planck-Institut fur Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany.
authors.initials: J
authors.lastname: Reichert
authors.initials: M
authors.lastname: Niering
authors.initials: R
authors.lastname: Holzwarth
authors.initials: M
authors.lastname: Weitz
authors.initials: T
authors.lastname: Udem
authors.initials: TW
authors.lastname: Hansch