Phase-coherent frequency measurement of the Ca intercombination line at 657 nm with a Kerr-lens mode-locked femtosecond laser

Abstract

The frequency of the calcium ${}^{3}{P}_{1}{\ensuremath{-}}^{1}{S}_{0}$ intercombination line at 657 nm is phase-coherently measured in terms of the output of a primary cesium frequency standard using an optical frequency comb generator comprising a sub-10-fs Kerr-lens mode-locked Ti:sapphire laser and an external microstructure fiber for self-phase-modulation. The measured frequency of ${\ensuremath{\nu}}_{\mathrm{Ca}}=455 986 240 494 276$ Hz agrees within its relative uncertainty of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ with the values previously measured with a conceptually different harmonic frequency chain and with the value recommended for the realization of the SI unit of length.