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Journal of the Optical Society of America B

Improving the sensitivity of optically pumped magnetometers by hyperfine repumping

V. Schultze, Theo Scholtes, R.P.J. IJsselsteijn, H.‐G. Meyer

Published Apr 2, 2015• 20 citations

Abstract

Most optically pumped magnetometers based on alkali atom vapor cells and pumped by a single narrow-band laser suffer from a loss of signal since atoms become trapped in the ground hyperfine states that are not coupled to the laser beam. This can be counteracted by additional optical repumping of these ground-state levels. We study hyperfine repumping using cesium vapor cells with partly overlapped ground-state splitting due to their nitrogen buffer-gas filling. We implement two methods of repumping and compare them to the conventional case of F=4 pumping: F=3 repumping with an additional repumper laser and combined pumping/repumping in the light-narrowing mode, where a single high-power laser is tuned near the F=3 transitions. All these modes are investigated for two different methods of spin-phase synchronization: the Mx and intensity modulation methods. For both methods, any kind of repumping results in a clear improvement of the magnetometer sensitivity compared to no repumping, but in the Mx mode it is more pronounced (about 50 versus 200 fT/√Hz for a 50 mm3 Cs vapor cell). The mechanisms responsible for the distinct results in the different working modes are discussed.

DOI: 10.1364/josab.32.000730

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Most optically pumped magnetometers based on alkali atom vapor cells and pumped by a single narrow-band laser suffer from a loss of signal since atoms become trapped in the ground hyperfine states that are not coupled to the laser beam. This can be counteracted by additional optical repumping of these ground-state levels. We study hyperfine repumping using cesium vapor cells with partly overlapped ground-state splitting due to their nitrogen buffer-gas filling. We implement two methods of repumping and compare them to the conventional case of F=4 pumping: F=3 repumping with an additional repumper laser and combined pumping/repumping in the light-narrowing mode, where a single high-power laser is tuned near the F=3 transitions. All these modes are investigated for two different methods of spin-phase synchronization: the Mx and intensity modulation methods. For both methods, any kind of repumping results in a clear improvement of the magnetometer sensitivity compared to no repumping, but in the Mx mode it is more pronounced (about 50 versus 200 fT/√Hz for a 50 mm3 Cs vapor cell). The mechanisms responsible for the distinct results in the different working modes are discussed.

We demonstrate a way of operating an optically pumped magnetometer with miniaturized cesium cell using the light-narrowing effect. The magnetometer setup shows improvement of shot-noise-limited sensitivity (

42\phantom{\rule{0.16em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}

in a cell of only

9.3\phantom{\rule{0.16em}{0ex}}{\mathrm{mm}}^{3}

volume) due to the suppression of spin-exchange relaxation to a large extent and the use of a strikingly increased fraction of alkali-metal atoms for signal generation, working even in $\ensuremath{\mu}\mathrm{T}$ magnetic fields, by using only a single high-intensity laser beam both for pumping and probing of atomic spins.

For the experimental implementation of an optically pumped atomic magnetometer, the magnetic resonance signal with a narrow linewidth and a high signal-to-noise ratio (SNR) is required for achieving a high sensitivity. Using 795-nm laser as both the pumping and the probe laser, we compare the magnetic resonance signals from different rubidium atomic vapor cells and investigate the variations of magnetic resonance signals with temperature. Optimized magnetic resonance signal is achieved with a paraffin-coated rubidium atomic vapor cell. Then the 780-nm laser at rubidium D2 line is introduced as a repumping laser, and we explore the changes of linewidth and SNR of the magnetic resonance signal under different power of the pumping laser and the repumping laser. Owing to the 780-nm repumping laser beam, the signal amplitude of rubidium-85 magnetic resonance signal is improved remarkably because more rubidium-85 atoms are spin- polarized by the 795-nm pumping laser beam. At the same time, the linewidth of rubidium-85 magnetic resonance signal is roughly not broadened anymore. We realize a closed-loop optically pumped rubidium-85 atomic magnetometer with a bandwidth of ~1.2 kHz, and the sensitivity is calibrated to be ~245.5 pT/Hz1/2 only with the 795-nm pumping laser beam. Owing to the employment of the 780-nm repumping laser beam, the sensitivity is improved to be ~26.4 pT/Hz1/2 which is improved roughly by one order of magnitude. We also calibrate the measurement accuracy and deviation of a commercial fluxgate magnetometer by using the enhanced rubidium magnetic resonance signal.

When the ground state hyperfine splitting of alkali metal vapor atoms is well optically resolved, the spin coherence on one hyperfine sublevel can be generated directly or indirectly by pumping the same or the other sublevel respectively. We experimentally studied the pump power dependence of the field response of a Bell-Bloom magnetometer for the two pumping schemes in a paraffin coated 87 Rb vapor cell. We find that although the maximum field response is achieved by combining the two pumping schemes, indirect pumping alone can do nearly as good while being much simpler to operate. We have achieved a sensitivity of 100 fT/Hz 1/2 with a single indirect pump at room temperature.

For precision atomic magnetometry, inert buffer gas is included in alkali-metal vapor cells to significantly broaden hyperfine transitions, which facilitates optical pumping and reduces diffusive relaxation, while also providing non-radiative excited state quenching. We show low-buffer gas pressure (below 50 Torr) alkali vapor cells with resolved hyperfine manifolds can also yield high-performance magnetometers. For high polarization in

^{87}

Rb, we optically pump

F=2

states with narrow linewidth

σ_+

light, while tuning a probe beam to depopulate

F=1

states (

Δν= 6.8

GHz from

F=2

). The probe tuning then also provides

F=2

detection with high optical rotation and low probe broadening; we demonstrate top-bottom gradiometry, within a single 25 Torr, 0.5 cc cell, that yields an Earth's field free-precession magnetometer sensitivity of 18 fT/

\sqrt{\text{Hz}}

with a 1 kHz bandwidth, as well as RF magnetometer sensitivity of 12 fT/

\sqrt{\text{Hz}}

in a small band about 110 kHz.

A high-sensitivity optically pumped atomic magnetometer working in the geomagnetic range utilizing the light-narrowing effect and in-phase excitation is described. The setup is based on a simple pump–probe arrangement built around a Cs vapor cell whose active volume is 64 mm3. The transverse oscillating field is applied parallel to the probe beam to drive Zeeman resonance, and the in-phase component of the resonance signal is measured to determine the field. The sensitivity of the magnetometer is improved by pumping most atoms into the stretched state. Consequently, spin-exchange relaxation is suppressed, and a sensitivity of 0.1 pT/Hz1/2 in the range of 10 μT is achieved. This magnetometer has the advantages of large dynamic range, high performance of low-frequency stabilization, high response speed, and compact size. It can be used for many cutting-edge applications such as detection of magnetic anomalies.