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

The frequency of the Calcium ^3P_1--^1S_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 u_Ca = 455 986 240 494 276 Hz agrees within its relative uncertainty of 4 10^-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.