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arXiv:2601.08716 · Journal of Applied Physics

Portable single-beam atomic total-field magnetometer for stand-off magnetic sensing

Heonsik Lee, Hyunbeen Lee, Minseok Choi, Yoontae Hwang, Deok Young Lee

Published Jun 16, 2026• 0 citations

Abstract

Optically pumped atomic magnetometers (OPAMs) offer high sensitivity at room temperature and are increasingly considered for portable magnetic sensing in geomagnetic-field environments. Here, we report a handheld-scale, single-beam scalar 87Rb OPAM with a sensor-head volume of approximately 110 ml. The device operates in an all-optical Bell–Bloom configuration and uses digital lock-in, dispersive tracking of the 87Rb Larmor resonance, implemented with a hybrid electronics stack that combines in-house control hardware with commercial modules. A single frequency-modulated laser beam performs both pumping and probing without RF coils. All signal processing is realized in Python on a single-board computer paired with a commercial off-the-shelf data-acquisition module, enabling immediate deployment without dedicated signal-processing hardware. The magnetometer has an intrinsic in-band field sensitivity of approximately 21 pT/Hz, estimated from the lock-in dispersion slope, over a 0.1–30 Hz closed-loop in-band region with a digital-output rate of 200 samples/s. In an unshielded Earth-field deployment, we detect repeatable transient magnetic signatures from a controlled elevator motion sequence and quantify standoff observability over sensor-elevator distances from 1.25 m to 10 m. These results show that compact scalar OPAMs can provide bandwidth and range-resolved event sensitivity suitable for field-deployable magnetic anomaly detection and infrastructure monitoring in realistic geomagnetic environments.

DOI: 10.1063/5.0320364

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Optically pumped atomic magnetometers (OPAMs) offer high sensitivity at room temperature and are increasingly considered for portable magnetic sensing in geomagnetic-field environments. Here, we report a handheld-scale, single-beam scalar 87Rb OPAM with a sensor-head volume of approximately 110 ml. The device operates in an all-optical Bell–Bloom configuration and uses digital lock-in, dispersive tracking of the 87Rb Larmor resonance, implemented with a hybrid electronics stack that combines in-house control hardware with commercial modules. A single frequency-modulated laser beam performs both pumping and probing without RF coils. All signal processing is realized in Python on a single-board computer paired with a commercial off-the-shelf data-acquisition module, enabling immediate deployment without dedicated signal-processing hardware. The magnetometer has an intrinsic in-band field sensitivity of approximately 21 pT/Hz, estimated from the lock-in dispersion slope, over a 0.1–30 Hz closed-loop in-band region with a digital-output rate of 200 samples/s. In an unshielded Earth-field deployment, we detect repeatable transient magnetic signatures from a controlled elevator motion sequence and quantify standoff observability over sensor-elevator distances from 1.25 m to 10 m. These results show that compact scalar OPAMs can provide bandwidth and range-resolved event sensitivity suitable for field-deployable magnetic anomaly detection and infrastructure monitoring in realistic geomagnetic environments.

A scalar magnetic field sensor based on a millimeter-size Rb87 vapor cell is described. The magnetometer uses nearly copropagating pump and probe laser beams, amplitude modulation of the pump beam, and detection through monitoring the polarization rotation of the detuned probe beam. The circularly polarized pump laser resonantly drives a spin precession in the alkali atoms at the Larmor frequency. A modulation signal on the probe laser polarization is detected with a lock-in amplifier. Since the Larmor precession is driven all-optically, potential cross talk between sensors is minimized. And since the pump light is turned off during most of the precession cycle, large offsets of the resonance, typically present in a single-beam Bell–Bloom scheme, are avoided. At the same time, relatively high sensitivities can be reached even in millimeter-size vapor cells: The magnetometer achieves a sensitivity of 1 pT/Hz1/2 in a sensitive volume of 16 mm3, limited by environmental noise. When a gradiometer configuration is used to cancel the environmental noise, the magnetometer sensitivity reaches 300 fT/Hz1/2. We systematically study the dependence of the magnetometer performance on the optical duty cycles of the pump light and find that better performance is achieved with shorter duty cycles, with the highest values measured at 1.25% duty cycle.

We demonstrate a compact single-beam optically pumped magnetometer (OPM) employing a pure 87Rb microfabricated vapor cell with 500 Torr buffer gas. The residual magnetic field is actively minimized to below 400 pT through a combination of mu-metal magnetic shielding and internal compensation coils. We systematically characterize the OPM response as a function of laser detuning and optical intensity, and derive the magnetic-field sensitivity from the signal slope and noise amplitude spectral density. A best sensitivity of 11 fT/Hz^1/2 is achieved near the resonance transition at an optimal optical intensity, using a small cell volume of only 0.027 cm^3. Furthermore, a miniaturized OPM prototype was developed using a folded flexible printed circuit board (FPCB) to integrate the vapor cell, a compact heater, and magnetic-field coils into a cubic sensing head. Two coil configurations patterned directly on the FPCB were investigated. Under optimized operating conditions, the OPM prototype achieved a sensitivity of 66 fT/Hz^1/2. These results demonstrate that high-buffer-pressure single-beam OPMs can achieve excellent sensitivity, establishing a pathway toward compact and scalable arrays.

We investigate an all-optical scheme for high-sensitivity measurements of the Earth's scale magnetic field (

B

). The scheme is based on the Bell-Bloom technique with frequency-modulated light. A single elliptically polarized light beam is used both for pumping the alkali-metal atoms and probing Larmor precession of their spins under the external magnetic field. In contrast to many other high-sensitivity magnetometry schemes based on nonlinear polarization rotation of the light, the proposed approach enables the observation of magnetic resonance via changes in the light-beam ellipticity parameter. The approach has been validated in experiments with a vertical-cavity surface-emitting laser, irradiating a

0.125

cm

^3

cesium vapor cell filled with a buffer gas. In the current experimental conditions, the achieved sensitivity is estimated at

220

fT/

\surd

Hz under

B

\,\approx\,

50

μ

T, while the shot-noise floor corresponds to

\approx\,

20

fT/

\surd

Hz. The bandwidth is estimated at

\approx\,

1

kHz. The proposed simple and robust single-beam scheme is well suited for miniaturization and is therefore particularly promising for the development of compact, highly sensitive, low-power-consumption atomic magnetometers for a wide range of applications in the geomagnetic field.

A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.

We propose a scheme for a Bell–Bloom atomic magnetometer based on a single beam. In our approach, the light is periodically modulated synchronously with Larmor precession. As a result, the light that interacts with atoms is alternately polychromatic and monochromatic. When the light is polychromatic, the ±1st sidebands of the light are optically resonant with the atoms, and the atoms are polarized by pumping. When the light is monochromatic, a differential detection technique extracts the Faraday-rotation signal of the light, and the polarization of the light is probed. We demonstrate an experimental implementation of the Bell–Bloom magnetometer that achieves a sensitivity for magnetic-field detection of 0.25 pT/Hz1/2 at 1 Hz. Our scheme is suitable for developing compact, miniaturized magnetometers.

Atomic magnetometry was performed at Earth’s magnetic field over a free-space distance of ten meters. Two laser beams aimed at a distant alkali-vapor cell excited and detected the 87Rb magnetic resonance, allowing the magnetic field within the cell to be interrogated remotely. Operated as a driven oscillator, the magnetometer measured the geomagnetic field with ≲3.5 pT precision in a ∼2 s data acquisition; this precision was likely limited by ambient field fluctuations. The sensor was also operated in self-oscillating mode with a 5.3 pT/Hz noise floor. Further optimization will yield a high-bandwidth, fully remote magnetometer with sub-pT sensitivity.