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Physical Review A

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

Akira Ozawa, Yohei Kobayashi

Published Aug 23, 2012• 21 citations

Abstract

The two-photon transition of ${}^{87}$Rb ($5{S}_{1/2}\ensuremath{-}4{D}_{3/2}$, $5{S}_{1/2}\ensuremath{-}4{D}_{5/2}$) 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.

DOI: 10.1103/physreva.86.022514

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The two-photon transition of

{}^{87}

Rb (

5{S}_{1/2}\ensuremath{-}4{D}_{3/2}

,

5{S}_{1/2}\ensuremath{-}4{D}_{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.

We present an experimental and theoretical investigation of two-photon direct frequency-comb spectroscopy performed through velocity-selective excitation. In particular, we explore the effect of repetition rate on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{3/2,5/2}$ two-photon transitions excited in a rubidium atomic vapor cell. The transitions occur via stepwise excitation through the

5{P}_{1/2,3/2}

states by use of the direct output of an optical frequency comb. Experiments were performed with two different frequency combs, one with a repetition rate of $\ensuremath{\approx}925$ MHz and one with a repetition rate of $\ensuremath{\approx}250$ MHz. The experimental spectra are compared to each other and to a theoretical model.

We present the observation of the Rb two-photon spectrum directly excited by an Er:fiber laser comb in a hot atomic vapor system. The influences of the atomic velocity distribution on direct and sequential two-photon transitions are discussed respectively and a combination of corresponding spectral control techniques is developed to eliminate the Doppler broadening background. A direct frequency comb spectroscopy is obtained with high-resolution two-photon transition lines. These techniques would benefit spectroscopy measurement in hot atomic vapor systems, and pave the way for establishing a high-stability optical frequency comb in 1.5 $\ensuremath{\mu}$m optical communication bands based on the Rb two-photon transitions.

The method of Doppler – free comb – spectroscopy for dipole transitions was proposed. The numerical calculations for susceptibility spectrum for two-level system driving by strong counter propagating combs were made. The narrow peaks with homogeneous width arise on the background of Doppler counter. The contrast of these peaks is large for largest amplitudes of comb-components. Power broadening is increasing with increasing of field amplitudes. The amplitudes of peaks depend on the phase difference between carrier frequencies of combs. The spectral range of absorption spectrum is determined by the spectral range of comb generator and all homogeneous lines arise simultaneously. The spectral resolution is determined by the width of homogeneously –broadening lines. The physical nature of narrow peaks is in the existing of multi-photon transitions between manifolds of quasi-energy levels arising for different groups of atoms moving with velocities that satisfy to the resonant conditions 2kv= (n+l)Ω⁄j, where n, l, j-are integers and Ω - frequency difference between comb teeth.

The two-photon transition $5S\ensuremath{-}5P\ensuremath{-}5D$ in rubidium vapor is investigated by detecting the fluorescence from the

6{P}_{3/2}

state when the atomic system is excited by the combined action of a cw diode laser and a frequency comb. The cw laser plays a role as a velocity-selective filter and allows for sub-Doppler spectroscopy over a large spectral range including the

5{D}_{3/2}

and

5{D}_{5/2}

states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.