Portable Single-Beam Atomic Total-Field Magnetometer for Stand-off Magnetic Sensing

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 $^{87}$Rb 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 $^{87}$Rb 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. In an unshielded indoor environment, the magnetometer exhibits a noise floor below 6 pT/$\sqrt{\mathrm{Hz}}$ near 80 Hz and a measurement bandwidth of 200 Hz. 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.

Figures (9)

• Figure 1: Lock-in-amplified Faraday rotation signal measured using the sensor described in Fig. \ref{['fig:fig2']} as a function of laser modulation frequency. ( Further details of the sensor implementation are provided in Subsection C.) Blue dots show the raw sensor readout after subtracting its mean value. The orange curve is a fit to a dispersive Lorentzian, yielding a center frequency of $\omega_L = 444.5~\mathrm{kHz}$ and a linewidth of $\mathrm{FWHM} = 11.61 \pm 0.3~\mathrm{kHz}$. The gray dashed line indicates a local linear approximation about $\omega_L$, with slope $-9.6~\mathrm{V / kHz}$. The residual asymmetry of the dispersion feature suggests an additional background contribution consistent with a hole-burning (Bennett-structure) origin, as discussed by Budker, Yashchuk, and Zolotorev BudkerYashchukZolotorev1998.
• Figure 2: (a) Schematic of the portable single-beam atomic scalar magnetometer. The sensor head consists of a DBR laser, a collimation lens, a quarter-wave plate (QWP), an in-house fabricated $^{87}$Rb vapor cell with an integrated heater, a half-wave plate (HWP), a polarizing beam splitter (PBS), and a balanced photodetector (BPD). (b) Photograph of the assembled portable atomic magnetometer. The sensor has overall dimensions of 28.4 mm $\times$ 32.1 mm $\times$ 121 mm (approximately 110 ml). (c) Block diagram of the data acquisition and digital signal processing chain. The differential optical signal from the BPD is digitized by a commercial data acquisition (DAQ) device and processed on a single-board computer using Python-based digital lock-in detection to extract the magnetic-field signal. (d) Photograph of the in-house electronic control board used for laser current and temperature control, vapor-cell temperature regulation, and modulation signal generation. The board has overall dimensions of 126 mm $\times$ 126 mm $\times$ 52 mm and consumes approximately 5 W during sensor operation.
• Figure 3: Amplitude spectral density (ASD) of the measured magnetic-field signal. The data were collected in an unshielded environment over a duration of 5 s with a sampling rate of 200 Hz. The dashed line indicates the measured noise floor, which is below 6 pT/$\sqrt{\mathrm{Hz}}$ around 80 Hz.
• Figure 4: Histogram of magnetic-field fluctuations measured in an unshielded environment. The solid line shows a Gaussian fit to the distribution, indicating approximately Gaussian noise characteristics over the measurement interval.
• Figure 5: Experimental setup for measuring elevator-induced magnetic-field disturbances using an atomic magnetometer. (a) Schematic of the elevator shaft showing the elevator car, motor, steel cables, and counterweight, which constitute the primary sources of magnetic disturbances during elevator operation. The car and counterweight move in opposite directions along the shaft. (b) Photograph of the measurement environment and sensor placement relative to the elevator door. The horizontal separation distance $D$ is defined as the distance from the elevator door to the magnetometer location. The sensor was mounted on a non-magnetic support structure and operated in an unshielded indoor environment.