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Journal of Physics B Atomic Molecular and Optical Physics

Optical pumping spectroscopy of Rb vapour with co-propagating laser beams: line identification by a simple theoretical model

Aleksandar J. Krmpot, Mihailo D. Rabasović, Branislav M. Jelenkovic

Published Jun 14, 2010• 5 citations
Publisher

Abstract

In this paper the saturation spectra of rubidium vapour at room temperature, obtained with overlapped co-propagating laser beams, were examined. Unlike the standard saturation spectroscopy, here the transmission of the pump laser beam was detected. The pump laser was locked to an atomic transition of the D2 line, while the probe laser frequency was scanned in a wide frequency range. The pump and probe beams had approximately the same intensities; thus the probe laser can saturate transitions and contribute to optical pumping. This, together with Doppler broadening, leads to rich pump transmission spectra, with many lines appearing due to the interaction of lasers with atoms in different velocity groups. The advantages of this method are well-resolved structures and appearance of spectral lines on a flat, Doppler-free background. Agreement between experimental and theoretical results shows the usefulness of this simple model, based on the rate equations, for identification of lines and determination of relative contribution to the observed line intensity from atoms with different velocities. Theoretical spectra are a useful tool for the calibration of experimental spectra obtained by a nonlinear dependence of the laser frequency on the voltage applied to the piezo used for the laser diode frequency scanning.

DOI
10.1088/0953-4075/43/13/135402

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In this paper the saturation spectra of rubidium vapour at room temperature, obtained with overlapped co-propagating laser beams, were examined. Unlike the standard saturation spectroscopy, here the transmission of the pump laser beam was detected. The pump laser was locked to an atomic transition of the D2 line, while the probe laser frequency was scanned in a wide frequency range. The pump and probe beams had approximately the same intensities; thus the probe laser can saturate transitions and contribute to optical pumping. This, together with Doppler broadening, leads to rich pump transmission spectra, with many lines appearing due to the interaction of lasers with atoms in different velocity groups. The advantages of this method are well-resolved structures and appearance of spectral lines on a flat, Doppler-free background. Agreement between experimental and theoretical results shows the usefulness of this simple model, based on the rate equations, for identification of lines and determination of relative contribution to the observed line intensity from atoms with different velocities. Theoretical spectra are a useful tool for the calibration of experimental spectra obtained by a nonlinear dependence of the laser frequency on the voltage applied to the piezo used for the laser diode frequency scanning.
Abstract We theoretically and experimentally investigate the Doppler-broadened absorption spectrum of the D2-line (52 S 1/2 → 52 P 3/2) in thermal (room-temperature) Rb vapour over a wide range of probe intensity, probe diameter, and vapour cell temperature to understand the effect of hyperfine pumping and transit-relaxation on the absorption lineshape. We present a relatively simple but comprehensive five-level rate equation model which incorporates optical pumping of the atomic population into the other ‘dark’ ground hyperfine level (leading to absorption saturation), including contributions of the three closely spaced (within one Doppler linewidth) hyperfine levels in the excited state 52 P 3/2 (allowed by the dipole transition selection rules). D2-line transmission spectra predicted by our model show excellent agreement (rms error < 5 % ) with the experimental data for a wide range of probe intensities (from ∼ 0.001 I s a t 0 all the way up to ∼ 10 I s a t 0 , where I sat0 is the usual two-level saturation intensity for the respective atomic transition), vapour cell temperatures (24.2°C–53.5 °C) and beam diameters (1/e 2 width ∼ 1 − 6 mm). Our model also takes into account the finite transit time of atoms across the probe beam cross-section, and correctly predicts the dependence of effective saturation intensity on probe diameter (up to ∼ 10 × lower saturation intensity as compared to I sat0 for few-mm beam sizes). This ab initio five-level model can thus predict accurate Doppler lineshapes for any given experimental parameter set for a linearly polarized probe without any fitting parameters, and can be easily applied to any other atomic system by an appropriate change of the atomic constants in the rate equations.
The commonly quoted saturation intensity of rubidium (Rb) for D2 transitions is 16.7W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. This value, however, is only accurate for the cycling transition, which renders the Rb atom to resemble a two-level system. In practice, the effective saturation intensity is influenced by various factors, such as the polarization of light, optical pumping, and atom collision which can redistribute the population. As a result, it is crucial to experimentally determine the effective saturation intensity. In this context, we demonstrate a novel yet simple experimental method to measure the effective saturation intensities of Doppler-broadened transitions in atomic vapours. Our method only requires on-resonance absorbance data for various input laser intensities, which can be obtained with a tunable CW laser without the need for precise frequency locking. We applied this method to measure the effective saturation intensities of all four Doppler-broadened D2 transitions in natural rubidium (Rb) vapour (27.8%87Rb and 72.2 %85Rb{}^{\text{8}} {}^{\text{5}}Rb85Rb) atr=35∘Cr={35}^{\circ} \mathrm{C}r=35∘C. The results show that the effective saturation intensities for Rb range from 0.752 to 16.8W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. Notably, for certain transitions (specifically, repump transitions), the effective saturation intensities were smaller than 16.7 W.m−2\mathrm{m}^{-2}m−2(up to an order of magnitude), with potentially important implications for optical pumping in Doppler-broadened transitions.
Abstract Laser spectroscopy of hot atomic vapours has been studied extensively. Theoretical models that predict the absolute value of the electric susceptibility are crucial for optimising the design of photonic devices that use hot vapours, and for extracting parameters, such as external fields, when these devices are used as sensors. To date, most of the models developed have been restricted to the weak-probe regime. However, fulfilling the weak-probe power constraint may not always be easy, desired or necessary. Here we present a model for simulating the spectra of alkali-metal vapours for a variety of experimental parameters, most distinctly at intensities beyond weak laser fields. The model incorporates optical pumping effects and transit-time broadening. We test the performance of the model by performing spectroscopy of 87Rb in a magnetic field of 0.6 T, where isolated atomic resonances can be addressed. We find very good agreement between the model and data for three different beam diameters and a variation of intensity of over five orders of magnitude. The non-overlapping absorption lines allow us to differentiate the saturation behaviour of open and closed transitions. While our model was only experimentally verified for the D2 line of rubidium, the software is also capable of simulating spectra of rubidium, sodium, potassium and caesium over both D lines.
We propose and demonstrate pump-probe spectroscopy of rubidium absorption that reveals the sub-Doppler hyperfine structure of the ${}^{5}{\mathrm{S}}_{1/2}\ensuremath{\leftrightarrow}{}^{5}{\mathrm{P}}_{3/2}$ (D2{\mathrm{D}}_{2}D2​) transitions. The counterpropagating pump and probe lasers are independently tunable in frequency, with the probe operating at the single-photon level. The two-dimensional spectrum measured as the laser frequencies are scanned shows fluorescence, Doppler-broadened absorption dips, and sub-Doppler features. The detuning between the pump and probe lasers allows compensation of the Doppler shift for all atomic velocities in the room-temperature vapor, meaning we observe sub-Doppler features for all atoms in the beam. We detail a theoretical model of the system that incorporates fluorescence, saturation effects, and optical pumping and compare this with the measured spectrum, finding a mean absolute percentage error of 3.11%. This high level of agreement allows us to extract the number density and temperature of the Rb\mathrm{Rb}Rb vapor from the two-dimensional spectrum. In the future this technique could be used as an atomic frequency standard for calibrating and characterising a single-photon or low-light-level source.
We have shown earlier that hyperfine spectroscopy in a vapor cell using co-propagating pump-probe beams has many advantages over the usual technique of saturated-absorption spectroscopy using counter-propagating beams. The main advantages are the absence of crossover resonances, the appearance of the signal on a flat (Doppler-free) background, and the higher signal-to-noise ratio of the primary peaks. Interaction with non-zero-velocity atoms causes additional peaks, but only one of them appears within the primary spectrum. We first illustrate the advantages of this technique for high-resolution spectroscopy by studying the D2D_2D2​ line of Rb. We then use an acousto-optic modulator (AOM) for frequency calibration to make precise hyperfine-interval measurements in the first excited P3/2P_{3/2}P3/2​ state of 85,87^{85,87}85,87Rb and 133^{133}133Cs.
The relative intensities of the Doppler-free signals of 85Rb and Cs D2 lines in saturation spectroscopy were calculated theoretically, taking into account the optical pumping process in a four-level system. The optical pumping effect in saturation spectroscopy is important when atoms with long-lived lower states are pumped by a weak beam. The theoretical results calculated from a simple analysis without any knowledge of the saturation parameters were found to be in good agreement with the experimental results on Rb and Cs D2 lines previously obtained using semiconductor laser.