Velocity-selective spectroscopy of Rb vapor with a train of short pulses and a diode laser

We perform a sub-Doppler spectroscopy of the D2 line of  87Rb atoms in a vapor cell using a mode-locked 1 GHz femtosecond laser and a cw diode laser. The experiment explores the high resolution of the frequency comb and the possibility of the atoms interacting with more than one mode of the comb. We probe several velocity groups or a selective group of atoms using a velocity-selective and a repetition rate spectroscopy, in a co-propagating configuration of the beams. By detecting the diode transmission we can distinguish the resonances associated with the hyperfine structure of the 5P3∕2 level and follow the competition of the optical pumping processes, in the regime that the two beams have approximately the same intensities. The results are well described in the frequency domain picture, using rate equations for a four level system interacting with three cw lasers.