Description
1. Introduction
To convert a femtosecond laser into a frequency-comb laser, it is frequently essential to use a costly atomic clock [1] 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].
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 [3] (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 [4]. In addition, the authors of reference [2] 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.
3. References
[1] R. Felder, Metrologia 42, 323-325 (2005)
[2] I. Barmes, S. Witte and K. S. E. Eikema, Phys. Rev. Lett. 111, 023007 (2013).
[3] C. S. Edwards, G. P. Barwood, H. S. Margolis, P. Gill, and W. R. C. Rowley, Metrologia 42(5), 464-467 (2005).
[4] 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 |
|---|---|
| How will you attend ICAP-27? | I am planning on in-person attendance |
