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In this paper, we present a simple scheme for efficiently removing the residual Doppler background of a comb laser based two-photon spectrometer to be better than 10−3 background-to-signal ratio. We applied this scheme to stabilize the frequencies of a mode-locked Ti:sapphire laser directly referring to the cesium 6S–8S transition and rubidium 5S–5D transition. We suggest a standard operation procedure (SOP) for the fully direct comb laser stabilization and evaluate the frequency of two spectral lines at a certain temperature, by which we demonstrate an all-atomic-transition-based Ti:sapphire comb laser merely via a 6-cm glass cell.

The clock transitions recommended by BIPM are never realized by a femtosecond comb laser. In this report, we recommend two comb-laser-based stepwise transitions resolved simultaneously from rubidium and cesium atom.

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The clock transitions recommended by the International Bureau of Weights and Measures (BIPM) are never realized by the ultrafast laser, owing to the insufficient spectral resolution and the difficulty of removing the residual Doppler background. Hence, investigations on the related frequency accuracy were not yet reported in literature. In other words, when a metrologist intends to suggest some clock transitions for locking an ultrafast mode-locked laser to become comb clock, three essential criteria must be satisfied: 1) high spectral resolution in the pulse train-atom interaction in which the target quantum state have been recognized by BIPM or have been proved to yield high frequency reliability, 2) pulse repetition rate and laser mode offset frequency are uniquely defined by the aforementioned target levels, 3) and possible error sources that would influence the transition frequencies are clarified. In this report we meet the above three criteria by using merely one 6-cm Cs-Rb glass cell, with which we not only demonstrate a compact scheme but also point out some intriguing physics in comb laser-atom interaction that benefits to verify the frequency accuracy.