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arXiv:physics/0307095 · Optics Letters

Saturated-absorption spectroscopy: eliminating crossover resonances by use of copropagating beams

Ayan Banerjee, Vasant Natarajan A

Published Oct 15, 2003• 58 citations
arXivHTMLPDF

Abstract

We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D2 line of 85Rb with a precision of 70 kHz and to resolve hyperfine levels in the D2 line of 39K that are less than 10 MHz apart.

DOI
10.1364/ol.28.001912

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Journal of Physics B Atomic Molecular and Optical Physics•Jul 28, 2020•11 citations
We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D2 line of 85Rb with a precision of 70 kHz and to resolve hyperfine levels in the D2 line of 39K that are less than 10 MHz apart.
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.
Doppler-free laser spectroscopy using polarization modulation has been carried out in Na D₁ and D₂ lines. The polarization of the pump beam is modulated between identically and orthogonally to a linear or circular polarization of the probe beam. The recorded signals of the hyperfine and crossover resonances are in agreement with a recent theoretical analysis of Nakayama which is based on velocity-selective optical pumping in a four-level system. Some exceptional discrepancy occuring in the D₂ is considered to be due to several relatively close lying resonances.
The goal of this experiment was to measure the hyperfine energy splittings of the ground to first excited state transitions in rubidium using saturated absorption spectroscopy. Using this technique, we measured these transition energy spectra by taking the difference of two photodiode outputs due to multiple beams of a single laser scanned over a range of frequencies and shone through a cell of Rb vapor. When the laser frequency was resonant with an atomic transition, photons of those frequencies were absorbed, leaving a dip in intensity of the beam measured at the photodiode. One of the two laser beams had its excitations saturated by a more intense, counter-propagating pump beam from the same laser. The pump beam saturated the absorption of the velocity equal zero atoms that the counter-propagating probe beam could have also interacted with due to no Doppler shift. At these frequencies the probe beam experienced less absorption that was measured by a photodiode. The photodiode output was calibrated to the change in frequency of the scanned laser using a Fabry-Pérot interferometer. The output of this interferometer served as a reference for how the laser frequency changed. Final results for the 87Rb F=1 spectrum accounted for the largest error with a maximum of 12.2% deviation from accepted energy spacing values due in part to systematic error. Other spectra measurements were in better agreement, possessing error ranging from 2.1% to 10.1% and some agreeing with accepted values to within our uncertainty.
A scheme of Doppler-free spectroscopy is experimentally demonstrated with a co-propagating control laser locking to an atomic hyperfine transition, and the differential transmission of the probe and the reference laser is detected. Crossover resonances are eliminated by selecting the class of atoms with zero velocity in the direction of beam propagation. In addition, the sub-Doppler spectrum experiences optical gain compared to the conventional saturated-absorption spectrum as a result of optical pumping.
Abstract An experiment on the saturated absorption spectroscopy (SAS) of 87Rb-D2 line is performed using the Laguerre–Gaussian (LG) beam as structured light. The theoretical simulation of which is done by considering a four-level atom-laser coupled system. The theoretically simulated spectra fairly match with the experimentally observed spectra. The line-width narrowing behavior of both the observed hyperfine and crossover resonances in the Doppler-broadened probe absorption spectra is attributed by the spatially dependent Rabi frequency of the pump LG beam irrespective of the characteristics of the probe beam. The results also illustrated that the narrowing behavior of the line shapes of the hyperfine structure using the LG beam is universal in nature, i.e. irrespective of hyperfine or crossover transitions. This structured light-induced narrowing of line-width of the sub-Doppler hyperfine and crossover resonances can be utilized for high-precision laser frequency locking at a particular hyperfine or crossover transition. Moreover, a significant narrowing of hyperfine and crossover resonances with the variation of the orbital angular momentum (OAM) number of the LG beam shows that the generated SAS signal can be manipulated by using the OAM of the structured light as a control knob.
Abstract We report on a global crossover saturated-absorption spectroscopy (SAS) of Cs D 2 transition using a dual-frequency laser field with a frequency difference of less than the Doppler width. With two pairs of dual-frequency laser beams counter-propagating through the vapor cell, the line shape and non-linear features observed in the transmission spectra of the probe beam change with the frequency difference of the dual-frequency laser. The superficially complicated transmission signal includes two sets of common single-frequency SAS and several velocity-selective resonances (VSRs), we call these VSR peaks ‘global crossover SAS’. This phenomenon is explained by VSR effect. The global crossover SAS is formed by the atomic velocity groups resonance with both the initial and modulated laser fields, which are counter-propagated. Moreover, the global crossover SAS dips having the separation of allowed hyperfine transitions move along with the two sets of common single-frequency SAS while the frequency difference of the dual-frequency laser is changed. This indicates a resonance between the velocity shifted pump laser frequency and the velocity shifted probe laser frequency. The results obtained in this system are of great interest for laser spectroscopy. Furthermore, the application of the counter-propagating laser beams in the dual-mode regime increase the number of atoms that effectively interact with the field, hence, this scheme has a potential advantage for laser frequency stabilization.