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 \nu_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. 
J. Stenger (1), T. Binnewies (1), G. Wilpers (1), F. Riehle (1), H. R.   Telle (1), J. K. Ranka (2), R. S. Windeler (2), A. ...