Treating cesium resonance lines with femtosecond pulse train

We present results of recent experiments when cesium vapor were illuminated by the fem-tosecond laser frequency comb centered at either D2 line at 852 nm or D1 line at 894 nm. This action changed the usual Doppler profile into very peculiar profile in which periodic structure can be observed by cw laser scanning probe. The periodic structure reflected the frequency spec-trum of the pulse train consisting of a series of fringes separated by the pulse repetition rate. In cesium system the atomic coherence relaxation time is longer than the laser pulse repetition period. Cs atoms interact with the spectrum of the pulse train, and not with the spectrum of a single pulse. This open up a possibility for the high resolution spectroscopy [1,2], where the observed linewidths are much less than the Fourier- transform limit of the individual pulse in the train. In our recent papers [3,4] we presented the observation of the velocity selective population transfer between the Rb ground state hyperfine levels induced by fs pulse train excitation. We developed a modified direct frequency comb spectroscopy (DFCS) which uses a fixed frequency comb for the 85,87Rb 52S1/2 → 52P1/2,3/2 excitation (Tsunami mode-locked Ti:sapphire laser with pulse duration of 100 fs and pulse repetition of 80 MHz) and a weak cw scanning probe (TOP