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Journal of Applied Physics

Optical pumping of a dense Na+He+N2 system: Application as an rf spectrum analyzer

A. C. Tam

Published Mar 1, 1979• 11 citations
Publisher

Abstract

An rf wave applied to an optically pumped Na vapor in a high-density He+N2 buffer gas, situated in a magnetic field gradient, can produce localized bright fringes due to local rf magnetic resonance. We examine in this paper the features of optical pumping in a dense alkali-helium-nitrogen vapor mixture, and investigate how such systems can be optimized for use as a simple real-time rf spectrum analyzer (for broadcast rf waves) with a wide frequency range (dc to several GHz). We derive the optimum values of the optical pumping rate, helium density, sodium density, and nitrogen density which are necessary in order to produce sharp, high-contrast, and bright fringes, and have accurate and unambiguous rf frequency spectra determinations with fast dynamic response. Samples of observed rf frequency spectra are presented, and the observed spectrum of a sinusoidal FM wave compares satisfactorily with the theoretical spectrum. The observed rf resolution bandwidth is about equal to the optical pumping rate, which is about 1 kHz in this study (rf frequency ∼1 MHz). Similar resolution can be obtained in the microwave range.

DOI
10.1063/1.326147

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Optical pumping is fundamental to high-precision measurement using thermal alkali-metal atoms in vapor cells. In applications such as atomic magnetometry, buffer gases (e.g., $\mathrm{N}_2$ or $\mathrm{He}$) at specific pressures are introduced to quench fluorescence and mitigate wall relaxation. In the high-pressure limit (e.g., the $\mathrm{N}_2$ pressure $p_{\mathrm{N}_2}> 1$~atm), where collisional broadening exceeds hyperfine splittings of the atoms, optical pumping theory provides a clear description of the angular momentum exchange between photons and atomic spins. However, in many magnetic sensing scenarios, the high-pressure approximation becomes inadequate as its pressure conditions are not strictly satisfied. Consequently, an explicit description of optical pumping under realistic pressures is critical for selecting operating points and enhancing system performance. To address this, we develop a unified theoretical framework of optical pumping in the quasi-high-pressure regime, where collisional broadening is comparable to the ground-state hyperfine splitting. We demonstrate that light absorption, spin polarization, and magnetic-resonance linewidth in this regime differ significantly from those predicted by the high-pressure limit and offer favorable operating conditions. Our study extends conventional modeling and offers critical guidance for atomic magnetometry operating under realistic buffer gas pressures.

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An rf wave applied to an optically pumped Na vapor in a high-density He+N2 buffer gas, situated in a magnetic field gradient, can produce localized bright fringes due to local rf magnetic resonance. We examine in this paper the features of optical pumping in a dense alkali-helium-nitrogen vapor mixture, and investigate how such systems can be optimized for use as a simple real-time rf spectrum analyzer (for broadcast rf waves) with a wide frequency range (dc to several GHz). We derive the optimum values of the optical pumping rate, helium density, sodium density, and nitrogen density which are necessary in order to produce sharp, high-contrast, and bright fringes, and have accurate and unambiguous rf frequency spectra determinations with fast dynamic response. Samples of observed rf frequency spectra are presented, and the observed spectrum of a sinusoidal FM wave compares satisfactorily with the theoretical spectrum. The observed rf resolution bandwidth is about equal to the optical pumping rate, which is about 1 kHz in this study (rf frequency ∼1 MHz). Similar resolution can be obtained in the microwave range.
We report on broad instantaneous bandwidth microwave spectrum analysis with hot Rb 87 atoms in a microfabricated vapor cell in a large magnetic field gradient. The sensor is a MEMS atomic vapor cell filled with isotopically pure Rb 87 and N 2 buffer gas to localize the motion of the atoms. The microwave signals of interest are coupled through a coplanar waveguide to the cell, inducing spin-flip transitions between optically pumped ground states of the atoms. A static magnetic field with large gradient maps the frequency spectrum of the input microwave signals to a position-dependent spin-flip pattern on absorption images of the cell recorded with a laser beam onto a camera. In our proof-of-principle experiment, we demonstrate a microwave spectrum analyzer that has ≈ 1 GHz instantaneous bandwidth centered around 13 GHz, 3 MHz frequency resolution, 2 kHz refresh rate, and a − 23 dBm single-tone microwave power detection limit in 1 s measurement time. A theoretical model is constructed to simulate the image signals by considering the processes of optical pumping, microwave interaction, diffusion of Rb 87 atoms, and laser absorption. We expect to reach more than 25 GHz instantaneous bandwidth in an optimized setup, limited by the applied magnetic field gradient. Our demonstration offers a practical alternative to conventional microwave spectrum analyzers based on electronic heterodyne detection. Published by the American Physical Society 2024
Abstract : A method for the optical observation of nutational resonance in optically pumped rubidium vapor has been studied both experimentally and theoretically. The method consists of pulsemodulating an rf magnetic field a one of the Zeeman resonance frequencies of the vapor and observing the resulting damped transient modulation in the transmitted light intensity. Of particular interest is the application of this method to a study of the spin relaxation of vapor atoms as influenced by the physical parameters of the system, in the case where the applied steady magnetic field is intense enough to cause individual Zeeman intervals to become noticeably unequal. Both single and double quantum resonances at a wide spectrum of Zeeman transitions of Rb-85 vapor diffusing through neon and helium buffer gases have been observed and identified, and the damping of the transient nutations in light intensity has been recorded under a variety of conditions. (Author)
A mixture of dense sodium vapor (number density [Na] \ensuremath{\sim} 1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$), and buffer gases helium, xenon, and nitrogen is optically pumped to a high degree of spin polarization using a single-frequency single-mode tunable dye laser. The relaxation in the "dark" of the variables 〈Sz〉〈{S}_{z}〉〈Sz​〉 and $〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉$ is studied as a function of [Na] and buffergas pressures. For lower helium pressures (46-torr helium and 2-torr Xe) the dominant contribution to the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in the presence of Xe comes from the "sticking" three-body collisions with He acting as the third body. This is further confirmed by studying the longitudinal magnetic field dependence of the spin-relaxation rate. At higher helium pressures (He 700 torr, and Xe 2 torr) the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in the presence of Xe is mainly due to the "sudden" binary collisions between the Na and Xe, and the relaxation rate of 〈Sz〉〈{S}_{z}〉〈Sz​〉 is independent of the longitudinal magnetic field. Significant hyperfine pumping is achieved by proper tuning of the laser. The temperature dependence of the relaxation of $〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉$ and 〈Sz〉〈{S}_{z}〉〈Sz​〉 is also discussed.
When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.
Cs and Rb atomic vapors have been optically pumped in their ground states. The optical/rf-field configuration used allowed for the optical detection of magnetic resonance spectra with subkilohertz resolution by a pump–probe beam technique, in which the free-precessional motion of spins is recorded in the time domain. A six-level system serves as a model to describe the dependence of the observed spectra on the optical frequencies involved. With an intense laser beam applied during the free-precession period, Zeeman light-shift effects are observed. The spatial electric-field distribution of the laser beams and particle diffusion have to be taken into account to let us explain the observed magnetic resonance line shapes adequately.
Optical pumping is fundamental to high-precision measurement using thermal alkali-metal atoms in vapor cells. In applications such as atomic magnetometry, buffer gases (e.g., N2\mathrm{N}_2N2​ or He\mathrm{He}He) at specific pressures are introduced to quench fluorescence and mitigate wall relaxation. In the high-pressure limit (e.g., the N2\mathrm{N}_2N2​ pressure pN2>1p_{\mathrm{N}_2}> 1pN2​​>1~atm), where collisional broadening exceeds hyperfine splittings of the atoms, optical pumping theory provides a clear description of the angular momentum exchange between photons and atomic spins. However, in many magnetic sensing scenarios, the high-pressure approximation becomes inadequate as its pressure conditions are not strictly satisfied. Consequently, an explicit description of optical pumping under realistic pressures is critical for selecting operating points and enhancing system performance. To address this, we develop a unified theoretical framework of optical pumping in the quasi-high-pressure regime, where collisional broadening is comparable to the ground-state hyperfine splitting. We demonstrate that light absorption, spin polarization, and magnetic-resonance linewidth in this regime differ significantly from those predicted by the high-pressure limit and offer favorable operating conditions. Our study extends conventional modeling and offers critical guidance for atomic magnetometry operating under realistic buffer gas pressures.