Kilohertz-resolution spectroscopy of cold atoms with an optical   frequency comb
  We have performed sub-Doppler spectroscopy on the narrow intercombination line of cold calcium atoms using the amplified output of a femtosecond laser frequency comb. Injection locking of a 657-nm diode laser with a femtosecond comb allows for two regimes of amplification, one in which many lines of the comb are amplified, and one where a single line is predominantly amplified. The output of the laser in both regimes was used to perform kilohertz-level spectroscopy. This experiment demonstrates the potential for high-resolution absolute-frequency spectroscopy over the entire spectrum of the frequency comb output using a single high-finesse optical reference cavity. 
T. M. Fortier, Y. Le Coq, J. E. Stalnaker, D. Ortega, S. A. Diddams,   C. W. Oates and L. Hollberg