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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Differential Detection of Doppler-Free Spectroscopy with Modulation Transfer

Tenghui Yang, Peter Yun

Published May 12, 2025• 0 citations
Publisher

Abstract

Combined with the modulation transfer method, we successfully observe a frequency discriminating signal with improved signal-to-noise ratio (SNR) in the scheme of differential detection of Doppler-free spectroscopy. In our previous counter-propagated pump-probe scheme, two orthogonal polarization components of the probe beam constrictive and destructive interact with dark state prepared by the pump beam, respectively. This led to simultaneous enhanced absorption and transmission by detecting the parallel and perpendicular polarization components of the transmitted probe beam. In this presentation, we add modulation to the pump beam, error signals are obtained by synchronous demodulating the probe beam transmission with the help of the four-wave mixing process in the light-atom interaction system. Our method not only boost the SNR of the error signal of differential detection signal compared to the traditional one, but also mitigate atomic resonance broadening and shift induced by direct modulation of the probe beam thanks to the modulation transfer spectroscopy. This new method may find applications in high-performance optical reference, high-resolution spectroscopy and so on.

DOI
10.1109/eftf/ifcs64367.2025.11194677

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We propose and implement a differential detection scheme of Doppler-free spectroscopy. By setting the proper relative polarization direction between the linearly polarized probe and pump beam, which are spatially counter-propagated and overlapped, simultaneous enhanced absorption and transmission are observed by detecting the parallel and perpendicular polarization components of the transmitted probe beam. The differential signal with improved signal-to-noise ratio (SNR) is also obtained, which is desired for compact optical reference, high-resolution spectroscopy and so on.
We propose and experimentally demonstrate a spectroscopic method, named full differential spectroscopy, that uses a single laser beam in retroreflected configuration to manipulate atomic dark states in a vapor cell. Conventional Doppler-free spectroscopy often suffers from limited performance due to the Doppler-broaden background. Here we show that by selectively coupling or decoupling the probe’s polarization components with the pump-induced dark state, we can simultaneously obtain both Doppler-free electromagnetically induced transparency and absorption. A differential signal, with enhanced Doppler-free amplitude and highly suppressed background and noise, is thus obtained. We systematically characterize how polarization angle and light power affect the resonance and noise properties. Using this approach, we stabilize a laser to a rubidium vapor cell, achieving a fractional frequency stability of 1.2 × 10 −13 at 1 s. These results demonstrate a simple, robust route to high-performance optical frequency references and may also benefit quantum sensing or high-resolution spectroscopy.
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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.
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Abstract This paper describes the first observation of polarization-selective four-wave mixing signals in conventional coupling-probe spectroscopy, specifically, saturation absorption spectroscopy in 85Rb atoms. The four-wave mixing signal is induced by two counter-propagating laser beams in a degenerate multi-level atomic system, involving the Fg=3→Fe=2,3F_g=3 \rightarrow F_e =2,3Fg​=3→Fe​=2,3 F g = 3 → F e = 2 , 3 , and 4 transitions of the 85Rb D2 line. Consequently, the four-wave mixing signals copropagating along the probe beam induce polarization rotation of a linearly polarized probe beam. To distinguish these four-wave mixing signals from the resulting probe beam, we detect the polarization components orthogonal to the polarization direction of the input probe beam, depending on the linear polarization angles between the probe and coupling beams. The experimental findings demonstrate excellent agreement with theoretical results.