Chirped-pulse direct frequency-comb spectroscopy of two-photon transitions

The two-photon transition of Rb-87 (5S(1/2) - 4D(3/2), 5S(1/2) - 4D(5/2)) in a vapor cell is investigated with direct frequency-comb spectroscopy. When strong chirp is imposed on a frequency comb, it is found that the Doppler-free signal amplitude barely degrades despite a significant reduction in peak intensity. In addition, it is found that the Doppler-broadened signal is strongly dependent on the applied chirp. We present a simple model that explains this counterintuitive behavior based on the elongated spatial extent of chirped pulses. Keeping Doppler-free signals almost constant, Doppler-broadened background signals can be strongly suppressed with chirped-pulse excitations. Consequently, in the case of uniformly distributed target atoms, chirped-pulse direct frequency-comb spectroscopy can be a useful method to improve the signal-to-noise ratio in two-photon direct frequency-comb spectroscopy.