To convert a femtosecond laser into a frequency-comb laser, it is frequently essential to use a costly atomic clock  to stabilize the repetition rate (f rep) and sophisticated nonlinear optics to lock the offset frequency (f ceo). In this report, we demonstrate that the mode frequencies of a comb laser can be directly referenced to two stepwise two-photon transitions (STPTs) in cesium and rubidium using just one 6-cm cesium-rubidium mixed cell and no extra clocks, revealing a new approach for comb laser frequency stabilization. The related spectra are based on a novel scheme for reducing the residual Doppler background in direct comb laser spectroscopy. For showing the possibilities of being a comb clocks, we investigate all potential error causes to demonstrate that our comb laser is capable of serving as a time-keeping equipment in the 778 nm to 822 nm wavelength range. The ultra-high spectral resolution (5 kHz) of our direct comb laser spectroscopy reveals some new and intriguing physics, such as the two-pathway interfered dip in rubidium spectra; the abnormal light shift and collision shift compared to what was observed by CW laser; a narrower linewidth despite the presence of broadening intermediate states; and the zero influence of the laser bandwidth, in contrast to the "strong-influence" conclusion in the direct two-photon transition experiment .
2. Experimental setup and result
Fig. 1 (a) depicts the schematic diagram of our experimental arrangement. The spectra concurrently resolved by our comb laser are shown in Fig. 1 (b). The two STPTs as the frequency references are line 7 in rubidium and line 8' in cesium, which are 5S1/2(F=1)>5P3/2(F=2)>5D5/2(F=3) and 6S1/2(F=3)>6P3/2(F=3)>8S1/2(F=3), respectively. To generate Doppler-free spectra, a piezoelectric transducer (PZT) in fig. 1 (a) was utilized to constantly change the overlapping condition of the laser beam, allowing only the fluorescence arising from the overlapped counter-propagating pulses to be amplitude modulated. In Fig. 1 (c), When compared to the CW laser experiment  (black line), the AC Stark shift of Rb line 7 (green line) indicates a completely different direction of detuning, although the slope of the comb laser light shift (red line) in Cs line 8' is close to that of the CW laser . In addition, the authors of reference  report a new error source in comb laser spectroscopy frequency accuracy; that is, the laser bandwidth or shaped pulses influence the atomic transition center as well as the linewidth in their comb-DTPT (direct two-photon transition) spectrum, whereas we did not find any such frequency shift in our STPT spectra, within our experimental resolution.
 R. Felder, Metrologia 42, 323-325 (2005)
 I. Barmes, S. Witte and K. S. E. Eikema, Phys. Rev. Lett. 111, 023007 (2013).
 C. S. Edwards, G. P. Barwood, H. S. Margolis, P. Gill, and W. R. C. Rowley, Metrologia 42(5), 464-467 (2005).
 C. M. Wu, T. W. Liu, M. H. Wu, R. K. Lee, and W. Y. Cheng, Opt. Lett. 38(16), 3186–3189 (2013).
|Presenter name||Liu, Tze-Wei|
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